SPATIAL AND TEMPORAL CHANGES IN PRECIPITATION IN PUERTO RICO FROM 1956-2010
A Thesis Presented to The Graduate Faculty of The University of Akron
In Partial Fulfillment of the Requirements for the Degree Master of Science
José Javier Hernández Ayala May, 2012
SPATIAL AND TEMPORAL CHANGES IN PRECIPITATION IN PUERTO RICO FROM 1956-2010
José Javier Hernández Ayala
Thesis
Approved: Accepted:
______Advisor Dean of the College Dr. Linda Barrett Dr. Chand Midha
______Co-Advisor Dean of the Graduate School Dr. Thomas Schmidlin Dr. George Newkome
______Faculty Reader Date Dr. John Szabo
______Department Chair Dr. John Szabo
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ABSTRACT
In this study annual, seasonal and monthly series for total precipitation are analyzed to find increasing or decreasing trends in rainfall over space and time in
Puerto Rico from 1956 to 2010. The precipitation series were observed at forty meteorological stations scattered over the island of Puerto Rico. The groups of series were selected for their spatial and temporal representativeness. In order to detect possible trends in precipitation over the island, the Mann-Kendall test was applied to the annual, seasonal and monthly series. This test is non-parametric and thus, has the advantage of being insensitive to the true (unknown) form of the distribution involved. The Mann-Kendall statistical test results of this research have shown that statistically significant increasing and decreasing trends occurred on some locations in the island of Puerto Rico from 1956-2010. The spatial and temporal distribution of the trends vary from increasing trends in the southwest, central and northeast regions for the annual series and dry season months to decreasing trends for the early wet season months mainly in the western area of the island. The results show that increasing trends occurred in the months of January, March, April, September and October mainly in the southwest and northeast and decreasing trends dominated in the months of
May, July, August, November and December mainly in the west.
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TABLE OF CONTENTS
Page
LIST OF FIGURES…………………………………………………..……………….vii
CHAPTER
I. INTRODUCTION…………………………………………………………………….1
Problem Statement…………………………………………………………….2
Importance of Study………………………………………………………...... 3
II. LITERATURE REVIEW………………………………...………………………….4
Global Changes in Precipitation Patterns………………………...…………4
Regions Impacted by Precipitation Change………………... ………………7
Precipitation Changes the Case of the Iberian Peninsula……………….10
Precipitation Changes in the Caribbean and Puerto Rico……………….11
III. DATA AND METHODOLOGY………………………..…………………………15
Area of Study…………………………………………………………………15
Statistical & Spatial Analysis………………………………………………..18
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IV. RESEARCH RESULTS……………………………………………….…………23
Spatial Distribution of Annual Rainfall Trends…………………………….23
Spatial Distribution of Early Dry Season Rainfall Trends………………...27
December Rainfall Trends…………………………………………..30
January Rainfall Trends……………………………………………..32
February Rainfall Trends……………………………………………36
Spatial Distribution of Late Dry Season Rainfall Trends……………...... 38
March Rainfall Trends………………………………………………..42
April Rainfall Trends………………………………………………....45
May Rainfall Trends………………………………………………….47
Spatial Distribution of Early Wet Season Trends…………………………49
June Rainfall Trends…………………………………………………52
July Rainfall Trends…………………………………………………..54
August Rainfall Trends………………………………………………56
Spatial Distribution of Late Wet Season Trends…………………………..58
September Rainfall Trends………………………………………….61
October Rainfall Trends……………………………………………..63
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November Rainfall Trends…………………………………………..64
V. DISCUSSION AND FINDINGS……………………………….…………………66
Annual, Seasonal and Monthly Rainfall Trends……………………….….67
ENSO Effects on Precipitation Trends in Puerto Rico…………….……..70
Cyclonic Precipitation Effects on Rainfall Trends in Puerto Rico……….74
Local Factors Effects on Precipitation Trends in Puerto Rico…………...75
VI. CONCLUSIONS………………………………………………………………….78
BIBLIOGRAPHY……………………………………………………………………...80
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LIST OF FIGURES
Figure Page 1.1 The Island of Puerto Rico and its main cities and elevation…………….16
1.2 Puerto Rico’s climate zones based on Koppen climate classification and selected weather stations……………...... 17
1.3 Average total annual precipitation for Puerto Rico from 1956-2010…...... 17
1.4 Monthly average precipitation for all stations in Puerto Rico from 1956- 2010……………………….…………………...18
2.1 Spatial distribution of Mann Kendall trends results for annual rainfall for 1956-2010……………………..…………….23
2.2 Annual rainfall totals for the weather stations in Paraiso, Juncos,AibonitoandVillalbafrom19562010………………………………..24
2.3 Annual rainfall totals for the weather stations in Ponce, Adjuntas, Corral Viejo and Yauco from 1956-2010………………………25
2.4 Annual rainfall totals for the weather stations in Ensenada, Lajas, Mayaguez City and Mayaguez AP…………………………………26
2.5 Spatial distribution of Mann Kendall trends results for early dry season rainfall for 1956-2010……………………………….…..27
2.6 Early dry season rainfall totals for the weather stations in Paraiso, Canovanas, San Juan and Aibonito…………………………….28
2.7 Early dry season rainfall totals for the weather stations in Ponce, Cerro Maravilla, Villalba and Isabela……………………….……..29
2.8 Spatial distribution of Mann Kendall trends results for December rainfall for 1956-2010………...…………………………………30
2.9 December rainfall totals for the weather stations in Aibonito, Villalba, Cerro Maravilla and Mayaguez City………………..….31
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2.10 Spatial distribution of Mann Kendall trends results for January rainfall for 1956-2010……………………………………………....32
2.11 January rainfall totals for the weather stations in Paraiso, Canovanas, Juncos and Trujillo Alto…………………………….33
2.12 January rainfall totals for the weather stations in San Juan, Rio Piedras, San Lorenzo and Aibonito……………………….34
2.13 January rainfall totals for the weather stations in Juana Diaz, Isabela, Maricao and Mayaguez City……………………….35
2.14 Spatial distribution of Mann Kendall trends results for February rainfall for 1956-2010………………………………………...36
2.15 February rainfall totals for the weather stations in Costanza and Lajas…………………………………………………………37
2.16 Spatial distribution of Mann Kendall trends results for late dry season rainfall for 1956-2010…………………………………38
2.17 Late dry season rainfall totals for the weather stations in Villalba, Cerro Maravilla, Ponce and Corral Viejo………………………..39
2.18 Late dry season rainfall totals for the weather stations in Adjuntas Sub, Yauco, Ensenada and Magueyes………………………...40
2.19 Late dry season rainfall totals for the weather stations in Lajas and Mayaguez AP…………………………………………………….41
2.20 Spatial distribution of Mann Kendall trends results for March rainfall for 1956-2010……………………………………………42
2.21 March rainfall trends for the weather stations in Paraiso, Aibonito, Guayama and Cerro Maravilla…………………………………..43
2.22 March rainfall trends for the weather stations in Ponce, Isabela, Yauco and Hacienda Constanza………………………………...44
2.23 Spatial distribution of Mann Kendall trends results for April rainfall for 1956-2010……………………………………………...45
2.24 April rainfall trends for the weather stations in Villalba, Yauco, Corral Viejo and Magueyes………………………………………..46
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2.25 Spatial distribution of Mann Kendall trends results for May rainfall for 1956-2010………………………………………………47
2.26 May rainfall trends for the weather stations in Jajome Alto, Maricao, Villalba and Mayaguez City……………………………………...48
2.27 Spatial distribution of Mann Kendall trends results for early rainfall season for 1956-2010…………………………………….49
2.28 Early wet season trends for the weather stations in Trujillo Alto, Villalba and Corral Viejo……………………………………...50
2.29 Early wet season trends for the weather stations in Maricao Fish, Coloso and Mayaguez City………………………………...51
2.30 Spatial distribution of Mann Kendall trends results for June rainfall for 1956-2010…………………………………………………52
2.31 June rainfall trends for the weather stations in Mayaguez City and Mayaguez AP…………………………………………53
2.32 Spatial distribution of Mann Kendall trends results for July rainfall for 1956-2010………………………………………………54
2.33 July rainfall trends for the weather stations in Villalba, Coloso, Mayaguez City and Mayaguez AP……………………………….55
2.34 Spatial distribution of Mann Kendall trends results for August rainfall for 1956-2010…………………………………………...56
2.35 August rainfall trends for the weather stations in Juncos, Guayabal, Jajome Alto, Corral Viejo and Mayaguez City……………….57
2.36 Spatial distribution of Mann Kendall trends results for late wet season rainfall for 1956-2010………………………………...58
2.37 Late wet season rainfall trends for the weather stations in Paraiso, Juncos, Gurabo and Aibonito…………………………………59
2.38 Late wet season rainfall trends for the weather stations in Villalba, Ponce, Mayaguez City and Mayaguez AP…………………..60
2.39 Spatial distribution of Mann Kendall trends results for September rainfall for 1956-2010………………………………………….61
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2.40 September rainfall trends for the weather stations in Rio Piedras, Mayaguez City, Mayaguez AP, Aibonito and Corozal…………………...62
2.41 Spatial distribution of Mann Kendall trends results for October rainfall for 1956-2010………………………………………….63
2.42 October rainfall trends for the weather stations in Gurabo, Cerro Maravilla and Mayaguez City……………………………..64
2.43 Spatial distribution of Mann Kendall trends results for November rainfall for 1956-2010………………………………65
2.44 November rainfall trends for the weather stations in San Juan, Villalba and Mayaguez City…………………………………66
3.1 Number of Station with significant precipitation trends in annual, seasonal and monthly series for Puerto Rico from 1956-2010…………68
3.2 Sum of all monthly stations with statistically significant rainfall trends by region in Puerto Rico from 1956-2010………………..69
3.3 Areas of Puerto Rico that experienced statistically significant trends for all of the twelve months combined from 1956-2010…………70
3.4 Early Dry Season trends and ENSO years from 1956-2010. Source: National Oceanic and Atmospheric Administration…………….71
3.5 January trends and ENSO years from 1956-2010.Source: National Oceanic and Atmospheric Administration………………………72
3.6 Late Dry Season trends and ENSO years from 1956-2010. Source: National Oceanic and Atmospheric Administration…………….73
3.7 April trends and ENSO years from 1956-2010. Source: National Oceanic and Atmospheric Administration………………………73
3.8 Annual rainfall trends and years when hurricanes pass through the island of Puerto Rico from 1956-2010………………………75
3.9 September rainfall trends and years when hurricanes pass through the island of Puerto Rico from 1956-2010……………………….75
3.10 June rainfall trends for the month of June in Mayaguez stations from 1956-2010……………………………………………………..77
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3.11 July rainfall trends for the month of June in Mayaguez stations from 1956-2010………………………………………..77
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CHAPTER I
INTRODUCTION
Global climate patterns have been changing in the last two centuries due to increasing surface temperatures and spatial redistribution of precipitation. It has not been established at what level or rate rising temperatures at the global scale spatially correlate with precipitation patterns change in some areas. Recent research suggests that changes in the earth’s atmosphere due to increasing amounts of greenhouse gases are warming the planet causing changes in spatial and temporal precipitation patterns (Dore, 2005). Recent studies on precipitation suggest that a small increase in global mean rainfall and latitudinal redistribution of precipitation has been caused by the warming of the earth’s atmosphere in the last two centuries. This warming of the atmosphere is causing changes in precipitation patterns in high latitudes where there has been an increase in rainfall. In the subtropical latitudes it has been causing decreasing values, and it has been possibly changing the spatial distribution of precipitation in the tropics by shifting the position of the Intertropical Convergence Zone. In this study I investigate the spatial and temporal changes in precipitation patterns in the island of Puerto Rico.
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Problem Statement
The island of Puerto Rico has been experiencing increases in mean average temperature associated with rising global temperatures and local factors such as urban heat islands (Gonzalez, 2005). It has also recently experienced changing precipitation trends in different areas of the island at different points in time (Peterson and Taylor, 2002). The aim of this research is to determine whether spatial distribution and temporal patterns of precipitation have significantly changed in Puerto Rico from 1956 – 2010.
This work aims to accept or reject two possible hypotheses. The null hypothesis of no significant changes in precipitation on the island over annual, seasonal and monthly time series and the alternate hypothesis of significant changes in precipitation, either increasing or decreasing in the island over annual, seasonal and monthly time series. If the null hypothesis is accepted it would be clear that precipitation patterns have not experienced significant change in the last 54 years. If the alternate hypothesis is accepted it will determine that significant changes in rainfall trends are occurring in the island, and it will also show what regions or parts of the island are where those changes are occurring.
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Importance of Study
It is very important to understand at what level spatial and temporal precipitation patterns have changed on the island because rainfall plays a very important role in Puerto Rico´s physical environment and society. Changes in spatial and temporal rainfall patterns would affect soils and vegetation and the physical and chemical composition of the coastal plains, forest, and wetlands of the island. This may cause some areas to become less favorable for certain species of plants and animals but more hospitable to others. In terms of the impacts on human activities in the island, precipitation changes may cause regions to experience more flooding and others to experience more droughts.
The changes will also affect water management in the island; places that depend on constant precipitation for their economic activities such as agriculture and industry will be directly impacted by changes in rainfall distribution.
From the water used for human activities such as agriculture, commerce, and industry to the role of water from rainfall as the actual physical agent in shaping the island´s landscape, understanding precipitation change over space and time is a key factor in planning the future of the island’s economic and environmental management. The intent of this research is to have a better understanding of precipitation changes over space and time in Puerto Rico.
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CHAPTER II
LITERATURE REVIEW
Global Changes in Precipitation Patterns
Understanding changes in spatial and temporal distribution of precipitation at the global scale is of crucial importance to all countries in the world because most socio-economic activities such as water management for agriculture, individual use, and power generation depend on this irreplaceable resource. Not only is it important to understand global changes in precipitation and their effects on society and economics, but also it is crucial to acknowledge the impacts it causes on the physical environment. Spatial redistribution of rainfall is causing desertification and droughts in regions of the planet that are experiencing decreases in rain and flood management issues in areas that are experiencing significant increases in precipitation.
Much discussion is being held on what is causing global precipitation changes in space and time, but most scientists agree that increasing surface temperatures are very likely to lead to changes in precipitation and atmospheric moisture causing changes in atmospheric circulation and a more active hydrological cycle (Dore, 2005). Other regional and local factors like “El Niño” southern oscillation (ENSO), topography, wind direction and urban heat islands 4
may also contribute to these changes in rainfall distribution in different areas of the planet (Gamble and Curtis, 2008). Precipitation patterns are affected not only by global factors, but regional and local factors may also affect rainfall distribution. Therefore, to acquire a complete perspective on the issue, one must consider all factors at all scales.
Since the beginning of the 20th century, global land precipitation has significantly increased by 2%, but this rainfall has been neither spatially or temporally uniform throughout the planet (Doherty, 1999). Recent studies show increases in total precipitation are closely linked to increases in heavy and extreme convective precipitation events caused by higher surface temperatures.
However in some cases regions that are not experiencing increases in rainfall may still experience an increase in the number of extreme precipitation event.
(Osborn, 2000).
On a global scale some regions are experiencing increases in precipitation and others are experiencing decreases. In the northern hemisphere where the major land masses are located, precipitation has mostly been increasing especially during the autumn and winter seasons, but these increases vary both spatially and temporally. In North America precipitation has been increasing by
5% in some areas and 10% in others since 1900. The increase has been most pronounced during the warm seasons, but has also been interrupted by multi- year anomalies such as droughts in the 1930s and 1950s (Groisman, 1999).
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Precipitation has been significantly increasing in some areas of the
Northern Hemisphere but other areas are experiencing decreases. One of these areas is China where there has been a slight decrease in annual precipitation, which also is shown by a decrease in number of rainy days (Ren, 2000). In the case of Europe, precipitation has been increasing in the north but decreasing south towards the Mediterranean (Hanssen and Forland, 2000). In the southern hemisphere researchers found that annual total precipitation has been increasing over much of Australia with significant increases of 15% to 20%. In Argentina a long term increase in precipitation has been observed from 1900 to 1998 (Dai,
1997).
Annual land precipitation has been increasing in the mid and high latitudes of the Northern Hemisphere from 0.5% per decade to 1% per decade with the exception of decreasing precipitation in Eastern Asia (Huffman and Adler, 1997).
In the subtropics land precipitation has been decreasing about 0.3% per decade since 1900, but recent measurements have shown increasing trends in the last decades of the century (Dore, 2005). In the tropics measurements show that precipitation has been increasing by 0.2% per decade over the 20th century. In contrast to the Northern Hemisphere, the Southern Hemisphere does not show a significant increase in annual total rainfall. Some areas of Argentina, Paraguay,
Brazil and Australia have been experiencing some increases in annual total precipitation and total number of rainy days, but no comparable systematic changes in precipitation have been detected in broad latitudinal averages over the Southern Hemisphere (Dore, 2005).
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Regions Impacted by Precipitation Change
The previous discussion was on changes in spatial and temporal patterns of global precipitation, but now the phenomenon is going to be observed at a regional scale, where the variability and change of precipitation over space and time is more related to the physical processes that occur in the oceans surrounding the individual regions, the direction of the winds, and the topography of the individual land areas (Dettinger and Cayan, 1998). Western North
America is an excellent example of a region involved in different physical processes that produce some interesting rainfall patterns. In this region precipitation tends to vary greatly over space and time. One of the most notable characteristics of this area is the contrasting variation of precipitation between the northwest and southwest; when one area is wet the other tends to be dry
(Cayan, 1998). Researchers were interested in understanding if the region was experiencing significant increases or decreases in precipitation. They examined rainfall and related climate variability in Western North America for a 110-year period and found that overall precipitation delivered in the area has stayed remarkably static for this period (Dettinger, 1998).
Other areas of the world such as Africa are experiencing changes in rainfall variability over space and time. On the African continent one of the most significant changes in climate variability has been the decline of precipitation in the Sahel region, which is the transition zone between the Sahara desert and the
Sudanian savannas (Nicholson, 1987). This area has been experiencing a severe drought since the 1960s, and the persisting drying trend is causing 7
concerns among government authorities regarding how to address losses of food production, episodes of food insecurity, displacements of populations, lack of water resources and constraints on hydroelectricity (Dore, 2005). These changes severely impact the physical processes of the area´s environment and also the different human activities in the areas. Impacts to the physical environment caused by changing precipitation patterns include desertification, droughts, loss of soil moisture and loss of ecological niches for endangered species (Serrano and Mateos, 1999).
While some regions are experiencing droughts and water shortages because of changes in spatial and temporal precipitation patterns, other regions are experiencing increasing trends in precipitation. One of the areas with significant change is the southeastern and northwestern regions of Australia, which are experiencing significant increases in total annual precipitation and frequency of heavy rainfall. Average rainfall has increased causing serious flooding issues (Haylock and Nicholls, 2000).
Changes in spatial and temporal patterns of precipitation are causing issues in both areas that are experiencing increases and areas that are experiencing decreases. For researchers it is of great importance to understand at what rate these changes are occurring to be able to assess and to predict which areas may suffer from the issues of increase or decrease in precipitation in the future. One of the regions that has been experiencing both effects of precipitation changes is Western Europe. In northern Europe precipitation has been increasing by 10-40% in the 20th century. In countries like Norway and 8
England autumn and winter precipitation has been increasing (Thomson, 1999).
When it comes to southern Europe, some areas have dried by as much as 20%.
For instance in Italy, total precipitation in the 20th century has decreased by 5% in the northern part of the country and by 15% in the south (Buffony, 2000).
Changes in spatial and temporal patterns in Europe are causing flooding in the northern countries and water shortage and even desertification in some parts of southern Europe.
Some new studies in different regions of the world suggest that parts where total amount of precipitation has been increasing are also experiencing more pronounced increases in heavy and extreme precipitation events (Haylock,
2000). In some areas of the globe, heavy rainfall events have been increasing even though the total amount of precipitation has decreased or remained constant (Dai, 1997). Some regions of the world are receiving greater amounts of rainfall in times when they used to receive less precipitation, and other parts are experiencing the opposite. In the countries located in the mid and high-latitude zones mainly in the Northern Hemisphere, there has been a statistically significant increase of 2% to 4% over the latter half of the 20th century in total precipitation derived from heavy and extreme rainfall event. (Dore, 2005).These statistically significant observed changes in precipitation in space and time are more than a signal of global climate change, they are a call for the scientific community to keep its eyes open and to understand the problems these changes in precipitation patterns may cause. Rainfall is being globally relocated by climate
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change, and perhaps those regions of the world who are least developed may be the ones that experience the most adverse consequences.
Precipitation Changes the Case of the Iberian Peninsula
One area having much research in determining spatial and temporal change in precipitation is the Iberian Peninsula. This area has been experiencing all kinds of problems related to precipitation surpluses and decreases, ranging from floods at some points in time in some parts of the Peninsula to droughts and even desertification in some areas. So scientists in the Iberian Peninsula and other parts of the planet are closely monitoring the changes in precipitation patterns that this area is experiencing and are trying to understand why these changes are occurring. It should be noted that historically, precipitation has been a climatic variable of great concern in northeast Spain because it has experienced a long term deficiency that has caused extreme drought events and desertification but also in other period’s catastrophic floods (Romero and Guijaro,
1998).
Recent research has concluded that there has been a significant change in spatial and temporal precipitation patterns in Eastern Spain; these are mainly successive drying in western Catalonia and central and western Andalucía (De
Luis and Raventos, 2000). In recent years with the increases in demand for water mainly for agricultural, industrial and tourism activities, precipitation spatial and temporal changes have become a major issue in eastern Spain.
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Other researchers have found that since the 1940´s the area has experienced decreases in annual total precipitation and as well as a significant downward trends in the month of March for almost the entire area (Serrano and
Mateos, 1999). This change in the spatial distribution of precipitation is causing serious issues like desertification in some parts of the peninsula and is forcing government authorities to enforce the creation of water management strategies that may partially relieve the enormous issue of water supply in the area.
Precipitation Changes in the Caribbean and Puerto Rico
Much research has been done on Caribbean climate, and many researchers have concluded that the region´s climate is as diverse as the islands that comprise the area. The climate in this region has been significantly changing over the last century in terms of temperature. The number of very warm days and nights is increasing and the number of dry days is decreasing, while the number of heavy rainfall events is increasing (Peterson and Taylor, 2002).
In terms of precipitation there are some general patterns that define the region, one of which is the wet season which spans late May through November.
The rainy season can be divided into two parts: an early season from May to July and a late heavy rainfall season from August to November (Gamble and Curtis,
2008). The heavy rainfall season receives more precipitation because of high hurricane activity in the region. A closer examination of the Caribbean precipitation records on the different islands indicates that there is significant interannual variability, some of which can be accounted for by variations in sea-
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surface temperature anomalies in the tropical Atlantic and Pacific (Taylor and
Enfield, 2002).
One of the main factors that influence precipitation variability in the
Caribbean is the phenomenon known as El Niño, where warmer waters off the
Pacific Coast of northwestern South America bring heavy rainfall to that area but not to the Caribbean (Taylor and Enfield, 2002). In the Caribbean, El Niño brings drier rainy seasons and wetter dry seasons whereas La Niña results in a wetter rainy season for the region. Therefore, this phenomenon must be accounted for when analyzing precipitation variability (Malmgreen and Winter, 1998).
A combination of rising temperatures, regional phenomena like El Niño and La Niña and high hurricane activity shows the complexity of precipitation patterns in this region. Furthermore, local factors related to the island’s geography like topography and urbanization also affect precipitation patterns. An excellent case is the island of Puerto Rico, the smallest island of the Greater
Antilles, where in a relatively small area of 3,515 sq. miles there are multiple climatic factors that are closely related to the island´s geography that result in a climatologically complex territory (Daly and Helmer, 2003).
In the case of Puerto Rico, climate phenomena are even more unpredictable because of the island´s small size. Recent studies have shown that
Puerto Rico is experiencing an increase in the number of warm days and nights and also is experiencing increasing trends in heavy rainfall events which correlates with what is happening in other parts of the Caribbean. The island of
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Puerto Rico has a rainy season that runs from late May to November, with
October being the rainiest month.
These rainfall patterns are the result of a combination of multiple climate factors like convective precipitation, cyclonic precipitation, rainfall from cold fronts and orographic precipitation (Malmgreen and Winter, 1998). The months of the year with less rainfall run from December to April, but the island can still receive considerable precipitation from cold fronts during those times. Precipitation is not evenly distributed across the island; if the island is divided into north and south considering the central mountains as the division line, we would find that the northern area of the island receives greater amounts of rainfall throughout the year than the southern part of the island.
On the island precipitation and cloudiness are strongly influenced by topography with mountainous areas being wetter than coastal areas. In terms of precipitation Puerto Rico is spatially very complex; the area of the island that receives the most annual precipitation is “El Yunque” National Forest in the northeastern part of the island. Not too far away in the southwest, we find the driest area known as the “Lajas” Dry Forest. The only way to understand Puerto
Rico´s complex climate is to consider all factors that affect the area from a geographic perspective, and from the atmosphere-lithosphere relationships to human-induced changes. A holistic view of how precipitation is spatially and temporally distributed in the island needs to be examined.
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Puerto Rico manages water sustainably, and like every other territory in the world it depends on water received from precipitation for most of its economic and social activities. In the last decade changes in spatial and temporal patterns of rainfall have been identified in the island. Updated measurements for 2010 have shown that this has been the rainiest year on record for the island (Miller,
2011). Global, regional and local changes in precipitation are occurring. Puerto
Rico may be also experiencing similar changes to those occurring in other areas on the world. That’s why is so important to try to understand climate changing trends in Puerto Rico, because the island´s geography is built around water.
In the physical environment, redistribution of precipitation may cause droughts in some locations that may affect the habitats of species at risk of extinction and may also provoke changes in the vegetated land covers. These changes can also cause flooding in areas where it did not historically flood. In terms of the human environment, slight changes in temporal and spatial patterns of precipitation may cause serious problems in social, agricultural, commercial and industrial activities on the island. Puerto Rico is surrounded by water and largely dependent on water from rainfall, so it is of great importance to understand at what level precipitation on the island is changing spatially and temporally. This allows government authorities on the island to plan for its future considering the change in precipitation.
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CHAPTER III
DATA AND METHODOLOGY
Area of Study
The area of study is the main island of Puerto Rico which is located at
18.5 ˚ N and 66.5˚ W covering an area of 3,515 sq. miles near the Greater
Antilles islands of Cuba, Hispaniola and Jamaica. It is bounded on the north by the Atlantic Ocean and on the south by the Caribbean Sea (Figure 1.1). Puerto
Rico is located in the tropic of Cancer in the path of the northeast trade winds which affect significantly the climate of the island. The island’s location makes it vulnerable to hurricanes from May to November. Puerto Rico is mostly covered with mountains and hills in the central region and with plains near the coasts. The island is divided between north and south by the central mountains. This division caused by the mountains causes a significant effect in precipitation patterns on the island, mainly between the northern and southern regions. The capital city of
Puerto Rico is San Juan which is located in the northern coastal plain area of the island. The total population of the island is approximately 3.8 million. The metropolitan area of San Juan accounts for an approximate population of 2 million, more than 50%.
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Figure 1.1 The Island of Puerto Rico and its main cities and elevation.
In climatological terms the majority of the island is classified as tropical rain forest (Figure 1.2) under the Koppen climate classification system. The areas to the north and southeast of the central mountains are the ones classified as tropical rain forest (Figure 1.2). The western region of the island is classified as tropical monsoon and south of the central mountains the island is classified as tropical savanna. This different climate classifications show the effect the central mountains have on precipitation in the island, where the north tends to be very wet and the south very dry. On the island annual average precipitation ranges from 29.9 inches (760 mm) in the southern part of the island in “Lajas dry forest” to 169.2 (4300 mm) inches in the northeast in “El Yunque rain forest (Figure 1.3).
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Figure 1.2 Puerto Rico’s climate zones based on Koppen climate classification and selected weather stations.
Figure 1.3 Average total annual precipitation for Puerto Rico from 1956-2010.
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Figure 1.4 Monthly average precipitation for all stations in Puerto Rico from 1956-2010.
Puerto Rico has two main seasons, the dry and the wet seasons. The dry season runs from December to early May. During the dry season almost all months have less than 4 inches of total precipitation (Figure 1.4). February is the driest month in Puerto Rico with an average of 2.3 inches of rain for the month.
The rainy season begins in late May and continues until November (Figure 1.4).
During the rainy season the majority of the months have precipitation amounts above the 6 inches mark; only June and July had rainfall amounts of less than 6 inches. September and October are the wettest months on the island with almost
8 inches on average total rainfall mainly because of the combination of cyclonic precipitation over the island and intense orographic and convectional rainfall in the north, west and central regions. November is a transitional month between the rainy and the dry season with 6 inches on average rainfall.
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Statistical & Spatial Analysis
For this study 40 weather stations with complete precipitation data for the time period of 1956 – 2010 were selected. Precipitation data were obtained from the National Climatic Data Center, the National Weather Service at San Juan
Data Center, and The Southeast Regional Climate Center. A Global Moran’s I statistical test of spatial distribution was done to determine whether the stations were spatially clustered and it resulted in a p value of less than 0.05 showing significant dispersion. The time period of analysis was determined by temporal continuity of data from the weather stations in which 82.5% of the 40 stations
(33) have complete data from January of 1956 to December of 2010, the other
17.5% (7) start in January of 1960 and may end at December 2010 or two years before in December 2008.
In this study the Mann-Kendall non-parametrical statistical test was used to test for trends. Trends were identified as one of the following: significantly increasing trends, no significant trends, or significantly decreasing trends. This statistical test involves computing a statistic S, which is the difference between the numbers of pair wise differences that are positive and the numbers that are negative. If the S is a large positive value there is an increasing trend in the data; if it is a large negative value, there is a decreasing trend in the data. This test has the advantage of not assuming any special form for the distribution function of the data, while having a power nearly as high as their parametric equivalents.
This is why its use is highly recommended by the World Meteorological
Organization (Mitchell, 1966). 19
The Mann-Kendall rank statistic t is illustrated by the following example:
∑
Mann – Kendall Test Example Time 1988 1989 1990 1991 1992 No. of No. of Data 5 6 11 8 10 + Signs - Signs 5 + + + + 4 0 6 + + + 3 0 11 - - 0 2 8 + 1 0 8 2 Step 1. Null Hypothesis Ho There is no trend Step 2. Alternate Hypothesis Ha There is an upward trend Step 3. Test Statistic S = Total # (+) –Total # (-) = 8-2= 6 Step 4. a) Critical Value Assuming the Critical Value is 8 Step 4. b) P value The P value of 6 is 0.117 Step 5. a) Conclusion As S=6 < 8 (critical value), fail to reject the null hypothesis of no trend Step 5. b) Conclusion Since p value = 0.117 > 0.05 we fail to reject the null hypothesis
Figure 1.5 Mann-Kendall statistic and results example.
The MK test for increasing trends with a nominal rejection rate of 0.05 % was first applied to the annual total precipitation series. After the MK test for increasing trends was applied, an MK test for decreasing trends was done. This is done to determine significant upward or descending trends considering only the annual total precipitation of each station (Serrano and Mateos 1998). Next the tests were applied to seasonal total precipitation series in which the seasons 20
are divided as early dry (Dec-Feb), late dry (Mar-May), early wet (Jun -Aug), late wet (Sept-Nov). The seasonal trends may show precipitation variability through the seasons and also exhibit which of the seasonal series have experienced increasing, no trend or decreasing trends. In the third and last part of the statistical analysis the tests were applied to monthly precipitation data series to determine if there are months experiencing significant changes in increasing or decreasing trends. The focus of this research was to go from a broader year by year analysis to a more specific month by month analysis to examine potential changes at different spatial and temporal scales.
The spatial aspect of this research was done by mapping the different values that resulted from the Mann-Kendall statistical test. The Man-Kendall tests for upward and downward trends were applied separately to the time-series data.
To differentiate between upward, decreasing and no change in the trends a different value was assigned to the three different scenarios. Increasing trends have a value of (1), decreasing trends a value of (-1) and no change in the trends a value of (0). Using those assigned values an inverse-distance weighted interpolation was done on all maps to visualize any regional or local trends.
The first map is the annual total precipitation trends from 1956-2010. This map shows the areas of the island that have experienced significant changes in precipitation either increasing (+), no change (•) or decreasing trends (-). The second part of the spatial distribution of change is the maps of the individual seasons. These maps show how precipitation has significantly changed spatially from 1955-2010 during seasons. The last part of the spatial aspect of my 21
research was to map precipitation trends of the 12 individual months for the time period. A total of 17 maps starting with the annual, the seasonal, and finishing with the monthly total precipitation changes determined which areas of the island have experienced significant changes in precipitation trends.
22
CHAPTER IV
RESEARCH RESULTS
Spatial Distribution of Annual Rainfall Trends
The spatial distribution of the results of the Mann Kendall test for the annual series is shown in Figure 2.1. Of the forty stations in this study 30 % (12) show significant trends; and of these ten stations indicated increasing trends and two showed decreasing trends. Spatially the increasing trends are located in the northeast and southwest regions of the island. In particular, the southwest region exhibits six weather stations with statistically significant increasing trends. For the annual negative series, the spatial arrangement is irregular.
Figure 2.1 Spatial distribution of Mann Kendall trends results for annual rainfall for 1956-2010.
23
Figure 2.2 Annual rainfall totals for the weather stations in Paraiso, Juncos, Aibonito and Villalba from 1956-2010.
The stations for the annual rainfall series vary dramatically in their temporal trends, showing different scenarios when annual precipitation patterns began to change (Figure 2.2). Using data from the rain gauge at Paraiso, the trend seems to be increasing steadily from 1970´s to 2010. In the stations at
Juncos and Aibonito the increasing trends appear after 1995 and continue to
2010. The station at Villalba shows decreasing rainfall trends from the 1970´s to the early 90´s.
24
The series for the stations located in the southwest region of the island show consistent increasing trends (Figure 2.3). The stations at Ponce, Corral
Viejo and Yauco demonstrate increasing trends continuously from the 1970´s to
2010. The time series for the rain gauge located in Adjuntas shows increasing trends starting in the early 1980´s and continuing to 2010.
Figure 2.3 Annual rainfall totals for the weather stations in Ponce, Adjuntas, Corral Viejo and Yauco from 1956-2010.
25
Figure 2.4 Annual rainfall totals for the weather stations in Ensenada, Lajas, Mayaguez City and Mayaguez AP.
The series for the stations in Lajas and Ensenada in Figure 2.4 show increasing trends. The series for the stations located in the west at Mayaguez show very interesting trends. The rain gauge at the city shows decreasing trends starting from the 1980s until 200, whereas the station more to the north at the airport shows increasing trends starting at the mid 1990´s and continuing until
2007.
26
Spatial Distribution of Early Dry Season Rainfall Trends
Of the 40 stations, only 20% (8) show significant trends for early dry season (Dec-Jan-Feb) precipitation. Of these, five stations (12.5%) exhibit increasing trends and a total of three stations (7.5%) show decreasing trends.
Three of the stations showing increases are clustered in the northeast region of the island. The other two stations with significant positive trends are randomly located in the central south and south regions of the island (Figure2.5). Two of the stations with a significant decreases in precipitation are located in the central mountains, suggesting a small regional trend. The other station is isolated in the northwest region of the island.
Figure 2.5 Spatial distribution of Mann Kendall trends results for early dry season rainfall for 1956-2010.
27
Figure 2.6 Early dry season rainfall totals for the weather stations in Paraiso, Canovanas, San Juan and Aibonito.
The series for the stations in (Figure 2.6) located in the northeast region of the island show very similar patterns. The stations at Paraiso and Canovanas show a consistent increasing trend since the 1970´s to the mid 2000´s. The rain gauge located at San Juan shows a small but consistent increasing trend in its series. For the Aibonito series the trends are different. They illustrate a decreasing trend in the period from 1980 to 2000 but experienced a sudden large increase in precipitation after the year 2000.
28
Figure 2.7 Early dry season rainfall totals for the weather stations in Ponce, Cerro Maravilla, Villalba and Isabela.
The early dry season trends for the stations in the central, south and northwest regions of the island do not show a lot of similarities in their series. The increasing trend for the rain gauge located at the south in Ponce is not very clear because the precipitation totals from year to year are not very distinct from each other. The totals range from 0 to 5 inches in almost all of the series. The stations in Villalba and Cerro Maravilla exhibit consistent decreasing trends in the series.
Both rain gauges show consistent negative trends in their early rainfall season totals since the 1970´s to 2007 (Figure 2.7)
29
December Rainfall Trends
The December precipitation trend map (Figure 2.8) shows that 10% (4) of the forty stations exhibit statistically significant change in trends. The four stations showing significant changes are located in the central mountains and the southwest region of the island. The series with increasing trends is located in the central mountains area of the island. The three stations with negative trends are located in the central mountains and the southwestern region of the island. A regional pattern was found in the central mountains area of the island where two stations in proximity to each other exhibited declining trends.
Figure 2.8 Spatial distribution of Mann Kendall trends results for December rainfall for 1956-2010.
30
Figure 2.9 December rainfall totals for the weather stations in Aibonito, Villalba, Cerro Maravilla and Mayaguez City.
The time series for the station located in Aibonito shows a lot of year to year variability for December rainfall, and it shows a significant increasing trend more pronounced in the years from 2003 to 2010 (Figure 2.9). The two series with decreasing trends in the Central Mountains in Villalba and Cerro Maravilla exhibit similarities in the periods of low rainfall values. Both rain gauges recorded the years from 1993 to 2000 as years with almost 0 inches for December precipitation. The station in the west at Mayaguez city shows a decreasing trend in its time series.
31
January Rainfall Trends
The January precipitation change in trends map (Figure 2.10) shows that
30% (12) of the 40 stations exhibit statistically significant change in trends for the time period from 1956-2010. The ten (25%) series with increasing trends are located in the northeast, central, and western regions of the islands. Seven of the ten stations with positive trends show a regional pattern in the northeast area of the island. The other three series with positive trends are dispersed in different areas of the island. The two stations with negative trends are located in the south and western regions of the island.
Figure 2.10 Spatial distribution of Mann Kendall trends results for January rainfall for 1956-2010.
32
Figure 2.11 January rainfall totals for the weather stations in Paraiso, Canovanas, Juncos and Trujillo Alto.
The series for the stations of Paraiso, Canovanas, Juncos and Trujillo Alto show similar trends in total precipitation for the month of January, where the increases in the three stations are identified in the early 90´s and continue to
2010 (Figure 2.11). The differences in precipitation totals for these three stations are caused by the location of the rain gauges. Paraiso and Canovanas have higher values because they are in the “el Yunque” rainforest area, whereas the
Juncos and Trujillo Alto are not, so they show lower precipitation values.
33
Figure 2.12 January rainfall totals for the weather stations in San Juan, Rio Piedras, San Lorenzo and Aibonito.
For the stations in San Juan and Rio Piedras located in the metropolitan area of the island the series show similar increasing patterns, with also some years with significant variability for January precipitation. Both of the stations exhibit increasing trends in January rainfall (Figure 2.12). For the stations at San
Lorenzo and Aibonito the trends show an increase in rainfall totals since the late
1990´s.
34
Figure 2.13 January rainfall totals for the weather stations in Juana Diaz, Isabela, Maricao and Mayaguez City.
For the more spatially dispersed series the plotted graphs show different trends in their precipitation values for January (Figure 2.13). The station at the south in Juana Diaz shows a consistent negative trend. The series for Maricao shows an increasing trend and a lot of year to year variability in its series. The series in Isabela shows a decreasing trend. The station at Mayaguez exhibits a similar trend to the rain gauge at Juana Diaz, a steady decrease in already low precipitation values for dry January.
35
February Rainfall Trends
The February precipitation trends map (Figure 2.14) shows that 5% (2) of the forty stations exhibit statistically significant trends for the time period from
1956-2010. The series with increasing trends is located in the southwestern region of the island. The station with negative trends is located in the western region of the island. None of the statistically significant series show any regional pattern and there is no apparent relationship with their neighboring stations.
Figure 2.14 Spatial distribution of Mann Kendall trends results for February rainfall for 1956-2010.
36
Figure 2.15 February rainfall totals for the weather stations in Costanza and Lajas.
The series for the station at Costanza shows a decrease in trends more pronounced between the years to 1997-2007 (Figure 2.15).Those trends are similar to decreasing trends found in the Mayaguez City station for January
(Figure 2.13). The series for the station in the southwest in Lajas shows an increasing trend; where the years with the highest precipitation amounts go from
1981 to1998.
37
Spatial Distribution of Late Dry Season Rainfall Trends
Figure 2.16 Spatial distribution of Mann Kendall trends results for late dry season rainfall for 1956-2010.
The late dry season (Mar-Apr-May) precipitation trends (Figure 2.16) shows that 25% (10) of the forty stations show statistically significant changes in their precipitation patterns. The rain gauges with increasing trends sum for a total of nine (22.5%), and they are almost entirely located in the southwest region of the island, with the exception of one station located in the west. In terms of the negative trends there is only one (2.5%) station with statistically significant values and it is located in the central-south region of the island.
38
Figure 2.17 Late dry season rainfall totals for the weather stations in Villalba, Cerro Maravilla, Ponce and Corral Viejo.
The only series with decreasing trends for the late dry season analysis is
Villalba; the graph shows that the station has been experiencing descending trends since the 1970´s (Figure 2.17). The other stations Cerro Maravilla, Ponce and Corral Viejo show similar increasing trends in their series.
39
Figure 2.18 Late dry season rainfall totals for the weather stations in Adjuntas Sub, Yauco, Ensenada and Magueyes.
The Adjuntas substation series shows increasing trends in the periods of mid 1960´s, mid 80´s and mid 90´s. The rain gauges located in Yauco, Ensenada and Magueyes show similar trends in their series, where increasing trends started in the mid 1960´s and continued through 2005 (Figure 2.18).
40
Figure 2.19 Late dry season rainfall totals for the weather stations in Lajas and Mayaguez AP.
The series for the rain gauge located in Lajas shows similar trends to those of the more southern stations like Ponce, Yauco, Ensenada and Magueyes where increasing trends are clear from the 1960´s to almost 2005.The series for the rain gauge located in the western region of the island in Mayaguez airport shows increasing trends, with high peaks in the last years of records.
41
March Rainfall Trends
The March precipitation change in trends map shows that 20% (8) of the forty stations exhibit statistically significant change in trends (Figure 2.20). The six (15%) series with increasing trends are located in the northeast, central, and southern regions of the islands. The stations with positive trends do not show significant regional patterns in general, but five of the rain gauges that experienced increasing trends for March are located south of the central mountains region. The other positive trend station is isolated in the northeast area of the island showing no spatial relationship between the other positive stations. The two (5%) stations with negative trends are located in the west and northwest regions of the island. The stations with negative trends are exhibiting a regional pattern in the western part of the island.
Figure 2.20 Spatial distribution of Mann Kendall trends results for March rainfall for 1956-2010.
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Figure 2.21 March rainfall trends for the weather stations in Paraiso, Aibonito, Guayama and Cerro Maravilla.
All stations located in the central mountains region show a similar trend.
The series at Paraiso, Cerro Maravilla and Aibonito show consistent increasing trends in precipitation for March with the typical year to year precipitation variability found in the central mountains (Figure 2.21). The station located at the south in Guayama shows a consistent increase in rainfall total amounts.
43
Figure 2.22 March rainfall trends for the weather stations in Ponce, Isabela, Yauco and Hacienda Constanza.
The other two stations located in the south, Ponce and Yauco, show similar trends to those found in the station at Guayama, where a small increase in precipitation in the month of March has been experienced (Figure 2.22). For the stations on the west of the island, the trends are negative, in both Hacienda
Constanza and Isabela precipitation amounts have been decreasing consistently since the 1980´s and most significantly in the last 20 years of the time period.
44
April Rainfall Trends
The April precipitation trends map shows that 10% (4) of the forty stations exhibit statistically significant trends (Figure 2.23). For April all of the stations showing significant trends are located south of the central mountains region in the southern and southwestern areas of the island. The three series with increasing trends are located in the south and southwest regions of the islands.
The stations with positive trends show a regional pattern in that southwestern region. The station with negative trends is located in the central mountains regions of the island.
Figure 2.23 Spatial distribution of Mann Kendall trends results for April rainfall for 1956-2010 .
45
Figure 2.24 April rainfall trends for the weather stations in Villalba, Yauco, Corral Viejo and Magueyes.
The station located in Villalba is the only station showing decreasing trends for April in the entire island (Figure 2.24). The station in Corral Viejo located in the southern hills of the central mountain shows a lot of variability in its
April precipitation time series but it also shows a consistent increase since the
1980´s to 2003. The stations located in the south at Magueyes and Yauco exhibit similar trends as Corral Viejo.
46
May Rainfall Trends
The May precipitation trends map shows that 4 (10%) of the forty stations exhibit statistically significant trends (Figure 2.25). For May, three of the stations showing significant change in trends are located in the central mountains region of the island and one is located in the west. The series with increasing trends is located in the central-west area of the island. The three stations with negative trends are located in the central mountains and western regions of the island.
The stations with decreasing trends are not exhibiting any clear regional patterns because they are randomly located in respect to each other.
Figure 2.25 Spatial distribution of Mann Kendall trends results for May rainfall for 1956-2010.
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Figure 2.26 May rainfall trends for the weather stations in Jajome Alto, Maricao, Villalba and Mayaguez City.
The series for the stations in Jajome Alto and Villalba show year to year rainfall variability in their series, with both series exhibiting decreasing trends.
The stations located more to the west show very different trends than those more to the center of the island. The plotted series for Maricao shows a lot of rainfall variability for the month of May, going from almost 0 inches in one year to 20 inches in the next. The series also shows that more precipitation began to fall in the 1990´s and continued to 2005. In the station at Mayaguez City the trend is similar to the one found in Villalba with a significant decrease in May rainfall trends more pronounce since the 1990´s to 2010.
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Spatial Distribution of Early Wet Season Trends
Figure 2.27 Spatial distribution of Mann Kendall trends results for early rainfall season for 1956-2010.
The early wet season (Jun-Jul-Aug) precipitation trends map shows that
15% (6) of the forty stations exhibit statistically significant trends (Figure 2.27).
The two series with increasing trends are located in the northeast and central- south regions of the islands, showing no regional pattern. The four stations with negative trends are located in the central mountains and western regions of the island. The western stations with negative trends are exhibiting a regional pattern of early wet season decreasing trends in that part of the island.
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Figure 2.28 Early wet season trends for the weather stations in Trujillo Alto, Villalba and Corral Viejo.
The early wet season series for Trujillo Alto shows an increasing trend from the mid 1980´s and continuing until 2010 (Figure 2.28). The station located in Villalba, shows a consistent decreasing trend since the 1970´s until 2008. In the Corral Viejo station located in the southern central mountains of the island the series suggest a consistent increasing trend.
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Figure 2.29 Early wet season trends for the weather stations in Maricao Fish, Coloso and Mayaguez City.
The series for the early wet season trends in the western part of the island show very interesting trends (Figure 2.29). All of the stations exhibit decreasing numbers in total precipitation for the months that make up the early wet season.
Of the three stations that show decreasing trends Mayaguez city has the steepest trend going from 30 to 20 inches of fallen precipitation between 1960 and 1970 to 15 to 10 inches between 1997 and 2007, a significant decrease.
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June Rainfall Trends
Figure 2.30 Spatial distribution of Mann Kendall trends results for June rainfall for 1956-2010.
The June precipitations trends map shows that 5% (2) of the stations exhibit a statistically significant trends (Figure 2.30). For June, the two stations showing significant trends are located in the western region of the island in the city of Mayaguez. The series with increasing trends is located in the Mayaguez airport in a mostly rural area. The station with negative trends is located in the city of Mayaguez. The stations with increasing and decreasing trends are exhibiting a very interesting regional pattern in the west region of the island.
There the station north of the city is experiencing increasing trends, and the station located inside of the city is experiencing decreasing trends.
52
Figure 2.31 June rainfall trends for the weather stations in Mayaguez City and Mayaguez AP.
The series for the stations located in the municipality of Mayaguez show opposing trends. The rain gauge located inside the city of Mayaguez is showing
(Figure 2.31) a consistent decrease in rainfall for the month of June, which is more pronounce in the years from 1990 to 2007. The station located outside of the city in the airport is exhibiting an increase in rainfall for June, which has been more pronounced since the 1990´s to 2007.These two series are showing a very interesting pattern where the rain seems to have shifted from the city more to the north.
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July Rainfall Trends
The July precipitation trends map shows that 12% (5) of the forty stations exhibit statistically significant trends (Figure 2.32). For July, four of the stations showing significant trends are located in the western region of the island and one is located in the central mountains. The series with increasing trends is located in the western area of the island. The four stations with negative trends are located in the western and central mountains regions of the island. Three of the negative series are clustered in the west region of the island, where all of the decreasing stations are surrounding the only series with increasing trends.
Figure 2.32 Spatial distribution of Mann Kendall trends results for July rainfall for 1956-2010.
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Figure 2.33 July rainfall trends for the weather stations in Villalba, Coloso, Mayaguez City and Mayaguez AP.
The series in the central mountains in Villalba shows a consistent decrease in precipitation for the month of July; where the most pronounced decreases in total rainfall go from 1990 to 2010. For the stations located in the western area of the island the series show very interesting trends for July precipitation (Figure 2.33). The stations in Mayaguez City and Coloso show similar decreasing trends. In their plotted series, both exhibit a consistent decrease in July rainfall.
55
August Rainfall Trends
The August precipitation trends map shows that 12.5% (5) of the stations exhibit statistically significant trends (Figure 2.34). For August, the stations showing significant trends are located in the northeast, south-central and western region of the island. The series with increasing trends is located in the southern area of the island. The four stations with negative trends are located in the western and central mountains regions of the island. These rain gauges do not show any regional pattern, three of the stations are located in the central mountains and one is in the western area of the island in Mayaguez.
Figure 2.34 Spatial distribution of Mann Kendall trends results for August rainfall for 1956 -2010.
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Figure 2.35 August rainfall trends for the weather stations in Juncos, Guayabal, Jajome Alto, Corral Viejo and Mayaguez City.
The station located in the northeast at Juncos shows a consistent decrease in August rainfall for its series, which has been more pronounced since the 1980´s (Figure 2.35). The rain gauges with significant negative trends in the southern central mountains exhibit a similar trend. In the station at Corral Viejo also located in the southern central mountains a significant increase in August rainfall is occurring. The rain gauge located in Mayaguez is experiencing decreasing trends for August, just as it did for the months of May, June and July.
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Spatial Distribution of Late Wet Season Trends
The late wet season (Sept-Oct-Nov) precipitation trends map shows that
20% (8) of the stations exhibit statistically significant trends (Figure 2.36). The five series with increasing trends are located in the northeast central, central- south, south and western regions of the islands, showing a small regional pattern in the central northeast area of the island. The three stations with negative trends are located in the northeast, central mountains and western regions of the island.
Figure 2.36 Spatial distribution of Mann Kendall trends results for late wet season rainfall for 1956-2010.
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Figure 2.37 Late wet season rainfall trends for the weather stations in Paraiso, Juncos, Gurabo and Aibonito.
The time series for late wet season trends in Paraiso shows a significant decreasing trend in total precipitation that goes from almost 40 inches in the 60´s period to around 30 inches close to the year 2010. The stations at Juncos,
Gurabo and Barranquitas all show similar increasing trends. They also show year to year variability and precipitation values more extremely apart from each other, similar to other series located in the central mountains of the island (Figure 2.37).
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Figure 2.38 Late wet season rainfall trends for the weather stations in Villalba, Ponce, Mayaguez City and Mayaguez AP.
In Villalba the time series shows consistent decreasing trends since the
1960´s. The station located at the south in Ponce exhibits an increasing trend in precipitation totals for late rainfall season but it also shows a lot of year to year variability (Figure 2.38). When it comes to the two stations in the west, Mayaguez
City and Mayaguez Airport the trends are different. For the station in the city the graph shows decreasing trends in late rainfall season precipitation, and for the one at the airport the graph shows increasing trends, similar to the trends found in the early wet season months.
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September Rainfall Trends
Figure 2.39 Spatial distribution of Mann Kendall trends results for September rainfall for 1956-2010.
The September precipitation trends map shows that 12.5% (5) of the forty stations exhibit statistically significant trends (Figure 2.39). For September, the stations showing significant trends are located in the north, south-central and western region of the island. The four series with increasing trends are located in the northern and western region of the island and showing a regional pattern in the north and central region of the island. The station with negative trends is located in the western region of the island. The September series located in the west show similar spatial patterns to the ones of June and July where the stations at Mayaguez show different trends in such proximity to each other.
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Figure 2.40 September rainfall trends for the weather stations in Rio Piedras, Mayaguez City, Mayaguez AP, Aibonito and Corozal.
The plotted series in the station located at the North in Rio Piedras shows increasing trends. The rain gauges located in Aibonito and Corozal in the central mountains region of the island exhibit a lot of variability in precipitation totals for
September. Both stations do not show a specific time period from where the increasing trends started, but both recorded high rainfall amounts for the late
1990´s. The Mayaguez City and Mayaguez AP series exhibit similar trends to those found in the previous discussed months, where the station in the city shows negative trends and the station more to the north at the airport shows increasing trends.
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October Rainfall Trends
The October precipitation trends map shows that 7.5% (3) of the stations exhibit statistically significant trends (Figure 2.41). For October, the three stations showing significant trends are located in the northeast, central mountains and western regions of the island. The series with increasing trends are located in the northeast region and central mountains and do not show any regional pattern.
The station with negative trends is located in the western area of the island, similar to the spatial patterns found in the months from May through September where this particular area of the island shows a significant decrease in their respective precipitation totals.
Figure 2.41 Spatial distribution of Mann Kendall trends results for October rainfall for 1956-2010.
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The time series for the station located in the northeast area of the island in
Gurabo exhibits a small but consistent increase in rainfall trends for the month of
October (Figure 2.42). The series for the station in Cerro Maravilla shows an increase in October rainfall with also a lot of year to year variability like other rain gauges located in the central mountains. The station located in the West in
Mayaguez City exhibits its usual decreasing trend observed previously in all of the early and late wet season months.
Figure 2.42 October rainfall trends for the weather stations in Gurabo, Cerro Maravilla and Mayaguez City.
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November Trends
The November precipitation trends map shows that 7.5% (3) of the forty total stations exhibit statistically significant trends (Figure 2.43). For November, the three stations showing significant trends are located in the north east, central mountains and western regions of the island, similar to the month of September.
The series with increasing trends is located in the north east region of the island.
The two stations with negative trends are located in the central mountains and western region of the island, similar to the spatial distribution of the trends in the previously discussed months.
Figure 2.43 Spatial distribution of Mann Kendall trends results
for November rainfall for 1956-2010.
65
The time series for the station located in the north in San Juan shows some year to year variability, with some years receiving as much as 16 inches of precipitation for November and others as low as 2 inches. For this station the mid
1970´s and the late 2000´s show as the periods where high rainfall amounts where experienced (Figure 2.44). The series for the station in Villalba in the central mountains exhibits year to year variability in rainfall amounts, going from almost 20 inches in one year to 0 inches in another. The series also exhibit a significant decreasing trend, most pronounce in the late 1980´s and late 2000´s.
The station located in the west in Mayaguez continues to show the same decreasing trend that all the wet season months discussed previously exhibited.
Figure 2.44 November rainfall trends for the weather stations in San Juan, Villalba and Mayaguez City.
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CHAPTER V
DISCUSSION AND FINDINGS
Annual, Seasonal and Monthly Rainfall Trends
The annual total precipitation series shows that 10 (25%) of the stations experienced annual increasing trends and only two (5%) exhibited negative trends in their series, while the rest of the 28 stations showed no statistically significant trends (70%) (Figure 3.1). For the early dry season, the majority of the stations had no statistically significant trends; the ones that had trends exhibited five stations (12.5%) showing increasing trends and three stations (7.5%) showing decreasing trends. The late dry season shows that 9 (22.5%) of the stations had increasing trends and only one station had an increasing trend while the rest of the thirty stations (75%) showed no significant trend. The late dry season is the season in which the most statistically significant trends occurred for
Puerto Rico.
The early wet season shows 34 stations (85%) exhibiting no significant trends and six stations showing significant rainfall trends; four stations (10%) are showing decreasing trends and two stations (5%) showing increasing trends
(Figure 3.1). Of all the seasons the early wet is the one with the fewest stations showing statistically significant trends for Puerto Rico. The late wet season 67
shows only eight stations (20%) with statistically significant trends with five
(12.5%) showing increasing trends and three (7.5%) showing decreasing trends.
Figure 2.1 Number of Station with significant precipitation trends in annual, seasonal and monthly series for Puerto Rico from 1956-2010.
The monthly rainfall trends varied, but similarities were found for the months of January (25%), March (15%) and April (7.5%) during which increasing trends occurred in the majority of the stations that showed trends from 1956-
2010. The months of February and June have the same amount of stations with increasing and decreasing trends. The months of July and August exhibited more than 34 stations with no trends and four with decreasing trends (10%) and one with increasing (2.5%). In the results for September and October, 10% and 5% of
68
the stations exhibited positive trends in their series, but similar to all of the months the majority of the stations in the island showed no significant trend.
Figure 3.2 Sum of all monthly stations with statistically significant rainfall trends by region in Puerto Rico from 1956-2010.
The graph above (Figure 3.2) shows the number of rain gauges that resulted in statistically significant increasing or decreasing trends by region. Of all of the regions the one with the largest amount of stations reflecting increasing trends was the northeast with 10 (25 %) series reflecting positive trends and also
1 (2.5%) station exhibiting decreasing trends. The region that follows the northeast is the west where the decreasing trends tend to be more concentrated with 12 (30%) of the statistically significant series showing negative trends and 3
(7.5%) showing increasing trends. The central mountains region exhibits 9
(22.5%) series with decreasing trends and 8 (20%) series with increasing trends.
69
The majority of the stations in the north and south show that the same amount of stations have increasing and decreasing trends in their series. The results show that increasing rainfall trends occurred mainly in the northeast, southwest and central mountains regions of the island while decreasing trends tend to be more concentrated in the western and central mountains region of the island.
4 0 8
Figure 3.3 Areas of Puerto Rico that experienced statistically significant trends for all of the twelve months combined from 1956-2010. The regions that experienced statistically significant increasing or decreasing trends for the sum of all of the months for the time period from 1956-
2010 are the northeast, central mountains, the southwest and the west (Figure
3.3). In the northeast the trends are entirely positive. In the central mountains region of the island the trends vary from increasing in the western central mountains, to decreasing in the more central areas. The southwest area of the island had almost all of its series showing increasing trends in precipitation. The western and central mountains regions of the island are the areas showing decreasing trends.
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ENSO Effects on Precipitation Trends in Puerto Rico
A strong signal of climate variability in the tropics is derived from “El Niño”
Southern Oscillation (ENSO). Many investigations have shown that ENSO significantly influences climate in many parts of the world including the Caribbean
(Malmgreen and Winter, 1998). In the case of Puerto Rico it is known that ENSO influences temperature and precipitation patterns. During “El Niño” (the warm phase) years higher temperatures are recorded and an increase in precipitation totals for the months that compose the dry season is experienced. During “La
Niña” (the cool phase) years lower temperatures are recorded and an increase in wet season precipitation is experienced in Puerto Rico. The results of this research suggest what other researchers have found, precipitation trends and variability in Puerto Rico are possibly related to ENSO (Taylor and Enfield, 2002).
Figure 3.4 Early Dry Season trends and ENSO years from 1956-2010. Source: National Oceanic and Atmospheric Administration.
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Figure 3.5 January trends and ENSO years from 1956-2010. Source: National Oceanic and Atmospheric Administration.
The early dry season is composed of the months that go from December through February and is during “El Niño” years where the high precipitation peaks are reflected. These results are similar to the ones found by scientists of the National Weather Service at San Juan in their paper titled; the local impacts of ENSO across the northeastern Caribbean. In this research paper the results suggested a relationship between “El Niño” years and increasing rainfall trends for the dry season months. For the early dry season series (Figure 3.4) the majority of the years with high precipitation peaks are “El Niño” years with the years from 1994 to 1998 being the years where very high amounts of rainfall were experienced for these stations located in the northeast area of the island.
The January series (Figure 3.5) for the stations located in the northeast area of the island shows a strong relationship with ENSO years, where the high precipitation values correlate with “El Niño” years. The strong “El Niño” years 72
from 1993 to 1998 and from 2003 to 2010 correlate with higher rainfall totals amounts for January.
Figure 3.6 Late Dry Season trends and ENSO years from 1956-2010. Source: National Oceanic and Atmospheric Administration.
Figure 3.7 April trends and ENSO years from 1956-2010. Source: National Oceanic and Atmospheric Administration. The late dry season and April series exhibit a similar relationship with the
ENSO years as the early dry season series. The late dry season is composed of 73
the months of March through early May and is during “El Niño” years when more than normal precipitation amounts are reflected in the majority of the stations. For the late dry season series the majority of the years with high precipitation peaks are ENSO years, with the years from 1977 to 1983 and 2001 to 2007 being the years when above average rainfall was experienced for these stations located in the southwest area of the island. The April series for the stations located in the southwest area of the island shows a relationship with ENSO years, where those rain gauges exhibit high precipitation totals for April (Figure 3.7). The strong “El
Niño” years expanding from 1977 to 1983 and from 2001 to 2007 correlate with high total rainfall amounts for April.
Cyclonic Precipitation Effects on Rainfall Trends in Puerto Rico
Precipitation from tropical cyclones is a common phenomenon in Puerto
Rico for the wet season months of August, September and October. For the annual series for the stations in the northeast the increasing trends correlate with years when hurricanes affected the island (Colon, 2009). The annual series
(Figure 3.8) shows that almost all of the years when precipitation peaks were the highest were the same years where hurricanes impacted the island of Puerto
Rico (Gamble and Curtis, 2008). Of the late wet season months, September was the one that was most impacted by cyclonic precipitation. The high hurricane activity years of the late 1950’s and the late 1990’s correlate with the high precipitation amounts of the September series (Figure 3.9).
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Figure 3.8 Annual rainfall trends and years when hurricanes pass through the island of Puerto Rico from 1956-2010. Source: National Oceanic and Atmospheric Administration.
Figure 3.9 September rainfall trends and years when hurricanes pass through the island of Puerto Rico from 1956-2010. Source: National Oceanic and Atmospheric Administration.
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Local Factor’s Effects on Precipitation Trends in Puerto Rico
The results of this research have shown that increasing or decreasing trends in Puerto Rico are related to known climate phenomena such as ENSO and hurricane activity, but this doesn’t mean that those are the only factors affecting precipitation trends in the island. Other local factors such as temperature, urbanization, and topography and wind direction have to be considered to understand the whole dynamics of change in precipitation trends for Puerto Rico. Of all regions on the island, the western area is the one that probably shows the best example of how local factors affect precipitation patterns. The two weather stations located in Mayaguez show how these local factors are interrelated each other, where one station is experiencing decreasing trends and the other one is experiencing increasing trends. The scenario might be the following; daily sea breeze in the early wet season months enters the city where it warms then rises up and condenses and then moves north following the direction of the mountains and instead of falling in the middle of the city, the rain falls in the station out of the city more to the north. The results for Mayaguez have shown similarities with studies in Houston were researchers have found that urbanization affects precipitation distribution over space and time (Burian and Sheperd, 2005). The wind from the sea breeze, the high temperatures in the urban area and the topography of Mayaguez combined might be the reason there are opposite trends in such a small area.
The series in Mayaguez for the months of June and July show that the opposite trends are a recent phenomenon. The divergent trends are clear since 76
the early 1990’s through 2007 (Figure 3.10). The station at the city shows decreasing precipitation trends since the 1990’s, but the station more to the north out of the urban area shows an increasing trend in the last 10 years of precipitation records (Figure 3.11).
Precipitation Inches Precipitation
Figure 3.10 June rainfall trends for the month of June in Mayaguez stations from 1956-2010.
Precipitation Inches Precipitation
Figure 3.11 July rainfall trends for the month of June in Mayaguez stations from 1956-2010. 77
CHAPTER VI
CONCLUSIONS
The results of this research have shown that statistically significant rainfall trends have been occurring at some locations in the island of Puerto Rico for the time period of analysis over annual, seasonal and monthly series. Most of the stations in the island showed no statistically significant trend. Only a few stations over annual, seasonal and monthly time scales exhibited statistically significant trends in some areas of the island. The annual total precipitation series shows that 10 (25%) of the forty selected stations experienced annual increasing trends, mainly in the southwest region of the island. The months that experienced the highest number of stations showing increasing trends were January with 10
(25%), March with 6 (15%) and April with 3 (7.5%). That is the reason why both the early dry and late dry seasons are the time periods in which increasing trends dominated in the island. For January, the region of the island that experienced the majority of stations with increasing trends was the northeast. The months of
March and April show that the southwest region of the island was the area where increasing trends in precipitation dominated from 1956-2010. Both early and late dry season months show a relationship with ENSO years, where Niño periods exhibited wetter months than the Niña periods.
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The months of July and August exhibited more stations with decreasing trends, both with 4 (10%) stations showing negative and 1(2.5%) exhibiting increasing trends. The two areas of the island where decreasing trends occurred were the west and central mountains regions. In the west it is possible that local factors such as topography, winds and urban surfaces are affecting the trends, where in some areas decreasing trends are occurring very close to areas of increasing rainfall trends, such as in Mayaguez. In the results for September and
October, 10% and 5% of the forty stations with significant trends exhibited positive trends in their series. The significant trends for these two months were concentrated in the north, central and west regions of the island and possibly related to high cyclonic activity. The stations that showed statistically significant trends for the months of November (5%) and December (7.5%) were exhibiting decreasing trends in the north, central, west and south west regions of the island.
This research has shown that statistically significant increasing and decreasing trends occurred at some locations in the island of Puerto Rico from
1956-2010. The spatial and temporal distribution of the trends vary from increasing trends in the southwest, central and northeast regions for the annual series and dry season months to decreasing trends for the early wet season months mainly in the west and central area of the island. The results show that increasing trends occurred in the months of January, March, April, September and October mainly in the southwest, central mountains and northeast and decreasing trends occurred in the months of May, July, August, November and
December mainly in the west and central mountains. This research has found
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that statistically significant increasing and decreasing trends are occurring in different regions at different time scales in the island, now the questions for further research are; what other factors might be causing these trends?, and what are the impacts this changes might have on Puerto Rico’s physical and human environments? Are the trends found in this research also related to cold fronts impact on the Caribbean or the North Atlantic Oscillation (NAO)? How do increasing or decreasing trends in rainfall going to affect ecological niches of endangered species and how can it affect water collection by dams in the island?
These are only some of the problems that could be explored now that this research has proven that statistically significant trends occurred at some locations in certain time scales on the island.
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