The Relationship Between Extreme Rainfall and Flood Tendencies in Pemali- Basin, North Central

By

Fransisca Mulyantari and R. W. Triweko

International Symposium on Ecohydrology Kuta, Bali , 21-26 November 2005 International Symposium On Ecohydrology Kuta, Bali Indonesia 21-26 November 2005 P OCEEDINGS

P.E. Hehanussa, G.S. Haryani, H. Pawitan and B. Soedjatmiko

International Uydrological. Programme •' ' International Symposium On Ecohydrology

Kuta, Bali Indonesia 21-26 November 2005 Proceedings

P.E. Hehanussa, G.S. Haryani, H. Pawitan and B. Soedjatmiko

IHP Technical Documents in Hydrology No. 4

Regional Steering Committee for Southeast Asia and the Pacific UNESCO office, 2005 LIPI Press, Jakarta, Indonesia ©Asia Pacific Center for Ecohydrology-Indonesian Institute of Sciences (LIPI) All rights reserved. Published 2005

ISBN: 979-3673-70-2

Cataloging-in-Publication Data

International Symposium on Ecohydrology: proceeding I [edited by] P.E. Hehanussa, G.S. Haryani, H. Pawitan, B. Soedjatmiko. (Assitant Editor] Hadiid Agita Rustini vii +313 pp.; 21x29.7cm ISBN ... 979-3673-70-2 1. Hydrology-ecology

551.48

Technical editing by Ari Irawan, Agusto W.M, Sarwintyas Prahastuti. Assistant editing by Siti Kania Kushadiani and Prapti Sasiwi.

Published by LI.PI .Press, member of IKAPI Jl. Gondangdia Lama 39, Menteng, Jakarta 10350 Telp. (021) 314 0228, 314 6942. Fax. (021) 314 4591 e-mail: [email protected] LIPI [email protected] [email protected]. id bmrlipi @uninet.net.id PREFACE

This volume contains 46 papers presented at the International Symposium on Ecohydrology held in Bali, Indonesia, 21 - 22 November 2005. This event is organized in conjunction with four other adjoining events, the Regional Steering Committee Meeting of the IHP-UNESCO National Committees for the Asia Pacific Region, the 3rd Asia Pacific Training Workshop on Ecohydrology, AP-Friend chapter-2, and the rumual meeting of the Scientific Advisol)' Committee on Ecohydrology, all are held as parallel sessions in Bali, Indonesia, during 23-26 November 2005. The intemati<:mal symposiwn will organize paper presentations on eight topics related to developments in ecohydro l o~:,>y or"the Asia Pacific region. They are: o Ecohydrology, spatial plruming, land cover and land use changes o Erosion and sedimentation trails o Water quality and envirorunental sanitation o Climate variability and ecohydrology o Water, culture, and religion o Water policy ar1d good governance o Best management practices o Hydrology and water resources This volume contains 46 papers that are presented during the two days deliberations. Not all the papers can strictly follow the organizing committees guidelines for not more than seven pages, there are papers with more pages. Due to communication problems and the limited time, we were not able to cut or rewrite the papers to less than 10 pages and have kept as it was. This international meeting is held at an early stage of ecohydrology developments in the Asia Pacific region. For this, there is a wide range oftopics, dispersed and not well-distributed group of papers presentation, which was unavoidable. The outcome Of this Bali meeting is expected to select and decide for a better­ concentrated group of activity that could be plrumed for the coming two years. It is a real pleasure of having papers from various countries in the Asia Pacific region to be presented in this event. There is a wide range of climate variability in the region, topography and tectonic differences, island­ small island-continental countries, transboundacy river basins, different social and demographic background, as well as the physical stage of developments in the member countries. With this diverse background it is really expected that there will be cross countl)'/region enrichment in ideas, dialogue, and exchange of experiences. We hope that this initiative of an international ecohydrology meeting and its deliberations will bring benefits to many of us. Contacts could be chrumeled through the UNESCO umbrella and regional organization to facilitate the challenges and problems faced by scientists in the Asia Pacific region.

Dr. Jan Sopaheluwakan, Chairman, Indonesian Nat. Com. for HIP-UNESCO

iii CONTENT

Preface...... 111 Content...... v Setting Flow Regimes For Protection of in-River Values in New Zealand R.P. Jbbitt And B.JF. Biggs ...... -· ...... Suitability of Gray Water Treatments For a Sustainable Sanitation System Naoko Nakagawa, Masahiro Otaki, Tomoaki Jtayama, Masato Kiji, Katsuyoshi Jshizaki ...... 9 An Application and Assessment of The Urban River Restoration in The Anyang River ofKorea Samhee Lee, Hongkee Jee and Soontak Lee ...... 15 A Survey ofBiological Wastewater Treatment in Japan and Its Application Possibility in Indonesia Ami A. Meutia and Hiroshi Tsuno...... 21 The Ecological Effects of Existed Lancang River Cascade Dams Liu Heng, Geng Leihua, and .Jiang Beilei...... 31 Water Quality ofTypical Estuaries in Coastal Area of Central Vietnam . LeDinh Thanh and Pham Hung...... 37 Romoting Ecosystem Based Approach of Integrated River Basin Management for Living Rivers in China Xiubo Yu A.J.M Smits Yi Wang Lifeng Li...... 41 Sediment Toxicity of Cascade System Reservoirs in Gunawan Pratama Yoga, Yoyok Sudarso, and Tri Suryono ...... 51 Particulate Organic Matter Dynamics Dwing A Flood Event in An Alpine River (Tagliamento, Ne-Italy) Chihiro Yoshin;l.tra, Klement Tockner, Hiroaki Furumai, and Kuniyoshi Takeuchi ...... 59 Establishment ofEco1ogical Effect Assessment Index System for Hydropower Development Geng Leihua, Liu Heng andHuangChangshuo...... 65 Application of A Distributed Rainfall-Sediment-Runoff Model to The Upper Basin, Indonesia Takahiro Sayama, Kaoru Takara, and Yasuto Tachikawa ...... ·...... 71 Migration of Pollutants and Effect on Groundwater System at Pulau Burung Sanitary Landfill Site, Penang, Malaysia Mohd Tadza Abdul Rahman and Ismail A bus tan...... 77 Prediction for The Runoff of The Mow1tain -Pass Stations ofHexi Area in Gansu Province in China Chen Yuanfang, Li Juan, Liang Zhongmin and Dong Zengchuan...... 87 On Estimation of Extreme Rainfall Events Van-Thanh- Van Nguyen and Tan-Danh Nguyen ...... 93 Historical Background of China's Policy of 'Converting Farmland Into Lake Area' and Its Influence on Social Systems Around Dongting Lake Region Takeo Onishi, Junichi Yoshitani, Yicheng Wang. Xiaotao Cheng...... 99 Effect of The Model Selection on The Methane Migration Simulation in The Subsurface Aquifer Kenichiro Kobayashi and Kaoru Takara ...... 105 The Relationship between Extreme Rainfoll and Flood Tendencies in Pemali-Comal River Basin, North Fransisca Mulyantari andR. W.Triweko ...... ___ ...... 111 ~ Management of Water Resources------Risk Degradation Based on The Status of Water Quality, Bali Island Ratna Hidayat ... · ~ ...... ·...... 117 Removal of Nitrogenous and Ortho Phosphate Compounds in Tapioca Waste Water Manufacture By Surface and Subsurface Flows in Constructed Wetlands Awalina and Ami A. Meutia ...... -· ...... 123 Cross-Conelation between Soi and The Monthly Precipitation in South Korea and Fukuoka, Japan AkiraKawamura, Young-HoonJin, KenjiJinno, andHideoAmaguchi...... 131

v Assessment of Water Quality Degradation for The Citarum and Brantas Rivers Using A New Developed Water Quality Index System M. A. Fulazzaky ...... -· ...... 137 Preliminacy Basin Water Balance Model Estimation of Temporal Distribution of Dissolved Sediment Load Into Ratai Bay, Lampung Apip, M. Fakhrudin and Hidayat...... 143 Transboundacy Drinking Water Sources Management At Mount Ciremai, Province, Indonesia Hilanat Ramdan, Kooswardhono Mudikjo, Dudung Dant.sman, Hidayat Pawitan...... 157 Ecohydrology and Water Resources Utilization ofCidanau Basin Waluyo Hatmoko ...... __...... :...... 165 Flood Runoff Analysis Using Width Function Hidetaka Chikamori andAkihiro Nagai...... 171 Regionalized Flood Frequency Distributions and Parameters for Philippine Rivers Leonardo Q. Liongson...... 177 Water Balance Simulation Model and Watershed Vegetation Analysis of 'Embung', A Man Made Water Reservoir in Timor Island -East Nusa Tenggara Province (A Case Study OfEmbungs In Oemasi, Oelomin, And Oeltua, Kupang District) Wahyu Widiyono, B. Lidon, and R. Abdulhadi...... _ ...... 183 Impacts of Climatic Change on Ecohydrology ofBangladesh M. Habibur.Rahman andAbdu/lahAI-Muyeed...... 191 Groundwater Flow System of Basin Based on Subsurface Temperature, Stable Isotopes and Hydraulic Head RobertM. Delinom, Yasuo Sakura, Changyuan Tang, Akinobu Miyakoshi, AriefRachmat...... 197 Ecohydrological Water Use and Water Management in The Island of Bali, Indonesia Tadashi Tanaka ...... -.· ...... 207 "I Am Just Borrowing Water But I Will Return It In An Hour" How Balinese Fanners Negotiate Their Daily Use of Irrigation Water Stephan Lorenzen, Rachel P. Lorenzen, Dr. Pascal Perez...... 212 Dynamic Groundwater/Surface-Water Interactions in A Confined Alluvial Aquifer: Implications for Ecohydrology Kitchakam Promma...... 219 Removal of Cod From Acidic and Organic Rich Laboratory Wastewater Through An Experimental Constructed Wetland · Awalina, Senny Sunanisari, Endang Mulyana and Sugiarti...... 225 Enhancement of Shallow Aquifer Utilization Using Geophysical Method and Collection Galleries Bogie Soedjatmiko...... 231 Numerical Model Simulation of Single Canal Surface Water Flow in A Case ofDownstream Boundacy Condition Change Arwin Sabar and De~y Sutikno...... 235 River- Coastal Ecohydro1ogy of Madura Strait, , Indonesia SenoAdi ...... ·- ...... 241 Application of AHP-GIS Method To IdentifY Potential Polluting Areas in The Upper Citarum River Basin Eko W. lrianto, A . Yuasa, P. Sudjono, andA. Heriansyah...... 251 Evaluation of Fishpond Hydrology: A Proposed Multidisciplinacy Approach for Water Conservation in Jakarta- Bogor Hinterland Area Tjandra Chrismadha., Peter E. Hehanussa...... 255 Acid Deposition Effects on Inland Aquatic Environment Jgnasius D.A. Sutapa & Eka Prihatinningtyas...... 263 Sustainability and Security of Water Resources in Beijing: Management Perspectives Zongxue Xu, and Jingyu Li...... 269 Change ofPorosity ofBed Material With Riverbed Variation Masaharu Fujita, Daizo Tsutsumi and Muhammad Sulaiman ...... 273 Lake Paniai and Tage, Two Ancient Sister Lakes At The Main Water Devide of Papua Island, Indonesia P.E. Hehanussd, TjiptasmaraandFirdausAchmad...... 279 Heavy Flood Discharge Prediction for The 2004 Fukui Rainfall Disaster in Japan and Prediction in Ungauged Basins Yasuto Tachikawa, Ryoichi Takubo, Takahiro Sayama, andKaoru Takara...... 285 The Recharge Area of Jakarta Groundwater Basin Using Hydrothermal Profile Robert M Delinom, Rachmat Fa/ar Lubls, Yasuo Sakura...... 291 Coordinated Sediment Flushing in Wlingi-Lodoyo Reservoirs and The Study oflts Economics Benefits and Effect to Water Quality and Ecosystem in The Downstream Reaches Soekistijono'; Aris Hamanto; Fahmi Hidayat...... 299 Removal of Basic Dyes from Textile Effluent Using Rice Hull as Filter Media E.J. Wiloso, V. B'arlianti, A.H. Setiawan, Y Sudiyani, and T. Basuki...... 309

vii THE RELATIONSHIP BETWEEN EXTREME RAINFALL AND FLOOD TENDENCIES IN PEMALI-COMAL RIVER BASIN, NORTH CENTRAL JAVA

. Fransisca MuJyantari1 und R.W.Triweko1 1Resellrcher in Hydrology, Research Institute f or Water Resources - M inistry of Public Works; [email protected] 1Senior Lecturer, Department ofC ivil Engineering. Parahyangan Catholic Univel·sity

ABSTRACT

Increasing of flood magnitude causes an increase of flood risk which is now recognized as the big problel\1. in most part of the world. It is still debatable whether increasing of rainfall intensity or changes of catchment or both of them are the main cause of flood discharge. Therefore, extreme rainfall and flood discharge need to be investigated. The objective of this study is to analyze the presence of tendency in annual maximum daily rainfall, annual maximum 2-day rainfall, annual maximum 3-day rainfall and annual maximum flood in Pekalon~an region and to analyze the relationship between extreme rainfall and flood tendencies. This study based on daily rainfall data that has a period of more than 50 years which covers the last two decades from 57 rainfall stations and annual maximum flood data from 6 gauging stations. The methods used in this study are Grubb and Beck method to detect outliers, whereas Spearman method to analyze the tendencies and linear regression to see the changes of data series for every period of analysis. The result showed that only a few data series of 1-day, 2-day and 3-day annual maximum rainfall have significant increasing or decreasing trend using 95% confidence interval. Design rainfall with 20 years return period changed every decade which followed the direction of the tendency. There was no significant tendency of annual maximum flood with 95% confidence interval, but the data series changed every year when linear regression was applied. The tendency of rainfall and flood discharge do not indicate strong relationship. It means that the tendency of rainfall is not the major reason of the flood tendency, other factor such as land use changes may give the main contribution.

Key words: tendencies; relationship, outlier; extreme rainfall

INTRODUCTION any trend. Some literature stated that the trend of the extreme rainfall was not found, because its One of the problems that attract many random characteristic (Frei and Schar, 2001). In the researclers today is the phenomena of global USA, it was found a tendency for 1-day and climate change that influence extreme events such several-day rainfall (Karl and Knight 1998). as flood!> and droughts. The changes of extreme Different opinion related to the existence or rainfall variability, especially its intensity and not-existence of extreme rainfall changes has been frequency, wi\1 impact to the resulted flood. The raised during the last decades. This research was changes can be explained by a tendency analysis, done by considering the quality of data which both for its intensity and frequency. This study will include the last decade data. In addition, it will also focus on the tendency analysis of the annual study the changes of flood discharge during the last maximum daily rainfall intensity, for 1-day, 2-day, years and its relationship to the changes of rainfall. and 3-day rainfall. To learn the relationship This study used the case of Pekalongan region, in between the tendency of rainfall and the r-esulted the northern part of the Central of Java. flood, the tendency of annual maximum flood will The objective of the study is to analyze the also be analyzed. presence of tendency in annual maximum daily Some research about the tendency of rainfall, annual maximum 2-day rainfall, annual extreme rainfall had been done in several countries. maximum 3-day rainfall and annual maximum In China, increasing trends are found for extreme flood in Pekalongan region and to analyze the rainfall in the eastern and north-west regions, while relationship between extreme rainfall and flood a decreasing trends are found in the center, north, tendencies. These results may c.ontribute useful and north-east regions. In the Yangtze river basin, information to hydrologist who involves in design it was found an increasing trend of 10 - 20 % of hydrologic structure such as dams, culvert, and every 10 years (Yuging and Zhou, 2002). Hidalgo river bank. et al ( 2002), in his trend analysis of maximum daily rainfall ranked the data from the highest to the DATA lowest, which from the ftrst to the tenth rank, then it was compared to the annual rainfall. It was done, Data used in this research is the daily rainfall because the extreme rainfall itself was not indicated data from 239 rainfall stations, which are

Ill distributed evenly in the Pekalongan region, n northern part of the Central of Java. The data was 6*L(Dt*Dt) provided by Meterology and Geophysical Board _ _,_i=.:.;l'------R sp = 1 - - (I) (Badan Meteorologi dan Geofisika) and Water n*(n*n-1) Resources Management Office (Balai PSDA) of in which, Pemali-Comal River Basin. Those data can be R.p coefficient of ranking-correlation from processed with a criteria that the blank data _in two Spearman weeks period are less than 5 days w1th an n number of data assumption that the total rainfall of less than 0.5 D the difference between ranking and rank of observation mm are considered as no rainfall. Extreme rainfall chronological number of daily, 2-day, and 3-day annual maximum rainfall Statistical test was done using Student's t were used in this analysis. For the best result, distribution test as follow: series of data longer than 30 years of record were 0.5 used. Statistical test for homogenity with Mann­ n-2 Withney (1947) test and independence test using t t = R sp [ 1 - R * R ] (2) Wald and Wolfowitz (1943) resulted 111 rainfall sp sp stations passed the test. With a degree of freedom of v = n - 2 and Rainfall larger than 400 mm was found in level of significance of 5 %. The hypothesis used in some stations which influence the trend of the data this study was, if the series of data has a trend, then series. Therefore, a special treatment was done to the value ofR.P is larger or less than zero, and if the test those data. The first test is to check the daily data series do not have a trend then the value ofR,P rainfall data which might be an error in the data equal to zero. input. Next, to check whether in previous days they In this study, data series which has a were written as "0", it might be an accumulation of tendency using Spearman test are classified into 3 the unrecorded data during those days. The second levels. Data series has a strong tendency if it test was done by comparing rainfall data of the includes in the 95% confidence interval, a moderate nearby stations for the same period, whether they tendency if it includes in the 90% confidence have the same pattern of increasing rainfall for the interval, and a mild tendency if it includes in the day when a rainfall larger than 400 mm occured. If 85% confidence interval. there was not an error, then the data can be used. To check the influence·oftrend to the design Among Ill rainfall stations, the stations rainfall, a design rainfall analysis of the rainfall which have the new data during the period of 1980 stations was done with a trend every decade with a to 2003 was selected. The criteria used in this return period of 20 years. The return period selection were the continuous data with a blank corresponding to the observed storm of every data of less than 3 years, in addition to the data with decade will be estimated by fitting a Pearson Type length of record of more than 50 years. All data III distribution to the annual maximum daily series. have a blank during the period of 1940 and 1989. The choice of the distribution is not critical since The number of stations passed the test is 57 stations the interest here does not lie in the accurate with a length of data around 51 years to 109 years estimation of return period, but to get under­ and an average of 80 years. standing how it changes as different samples for The data used for annual maximum flood every decade. In addition, the existence of upp.er comes from 6 A WLR stations with a short record outlier in the data series was analyzed of tts of 10 to 26 years. One of the data should be influence to the existence of trend by neglecting the eliminated because of its fluctuation, several times value of outlier from data series and it was detected of discharge larger than 1000 m3Is. With a short the existence of trend. record, there is a difficulty in doing an analysis of Detection of outlier was done using Grubbs the existence of tendency in the data series that and Beck test as recommended by the United States cannot express the real condition, compared to the Water Resources Council (Bobee and Ashkar, tendency of rainfall. 1991). This test was based on the assumption that METHOD series of data follow a normal distribution. This outlier detection used the package of program The series of data were detected their Hydrological Frequency Analysis (HFA), where all existence of trends is the daily, 2-day, and 3-day data were converted to logarithmic and assumed to annual maximum rainfall of 57 rainfall stations follow normal distribution. If there was a lower which has been passed the statistical test with a outlier in the series of data, then the value was length of data of more than 50 years. While the eliminated, while the upper outlier was still maximum discharge data from 5 automatic water included in the data series. level recorders were used in this study. The method The relationship between extreme rainfall applied in this study was the linear regression to and maximum annual discharge was analyzed. observe the changes of direction in each decade and There are three possibilities from this trend analysis the Spearman method to detect the existence of results, (i) the existence of trend of rainfall, but not tendency using the following equation: for flood discharge or the reverse; (ii) both rainfall

11 ') and discharge are w.ithoul trend, or (iii) the rainfall consistently had a strong tendency, i.e. Station 15a has a positive trend while the discharge J1as a and 73. Data series of annual maximum daily negative trends or the reverse. rainfall for Station 73 with an increasing tendency using linear regression is shown in Fig. 2. Rl!sULTS AND DISCUSION Rainfall data series with an outlier were found in station number 54, 82a and 96a. Upper lrom 57 rainfall stations which has a outlier was excluded from the data series and the tendency with Spearman test using a confidence tendency was tested using Spearman method. The interval <)f two-sided test, in accordance with its result shows that there is no significant influence level. Obvlous\y the rejection region for the null from the outlier to the level of tendency, because hypothesis (no trend) is below 2.5% for a the decrease is only about 3%, except for the significant decreasing trend and above 97.5% for a decreasing tendency that increasing of one level. $igniflcant increasing trend. In Table 1, rain gauges Manton et al (2000) did a research about presenting tendency for annual maximum daily trends for extreme daily rainfall and temperature in rainfall, 2-day daily rainfall and 3-day maximum South East Asia and Northern Pacific, included maximum daily rainfall. Only 20 stations showed Indonesia. The result was that there was no tendency and others can be categorized as no significant trend for extreme rainfall in Indonesia. significant tendency. Rainfall stations with a He used data from 1961 to 1998 from the rainfall positive tendency located in the eastern part and the stations in Pangkal Pinang, Jakarta, Balikpapan, one with a negative trend located near the western Menado, Ambon and Palu. This result is in part, while the western part there is significant trend accordance with the result of this study, that only (see Fig. 1). 10% of the rainfall stations has a strong tendency For the maximum annual daily rainfall, there .for annual maximum daily rainfall. were 6 stations with an increasing (positive) Design rainfall analysis with a return period of tendency Qr 10% of the total 11\lntber of station, and 20 years for every decade was done in each rainfall a station (or 1.75%) with a decreasing (negative) stations, using the method of linear regression to know trend with a confidence interval of 95%. Other the changes according to the average. The result of the stations had an increasing and decreasing trend with analysis in each station indicates the increase or a confidence interval of 90% (moderate tendency) decrease of the value of design rainfaH in every and 85% (mild tendenc-y), For the 2-day annual decade in accordance to the Spearman test. The maximum rainfal~ there was only 4 stations (7%) average of angle resulted from the linear lines of the which had a trend and 3-day annual maximum tendency of the linear regression compared to their rainfall, there was only 3 stations (5%) had a trend. average are larger than so• except for station 48a There was a difference in the level of tendency which is smaller than 50o. It means that the changes of between 2-day and 3-day rainfall compared to 1- the design rainfall in every decade are significant; day rainfAll, and even some stations do not indicate either it has strong, moderate, or mild tendencies. a tenden:::y. There were two rainfall stations which Table 1. Tendency test result. Ave. change*) mm/10 No N~. Sta. Number of data 1-day 2-day 3-day Angle*) a• years 1 sa 70 ++ + + 3.50 74.06 2 15a 71 +++ +++ +++ 1.97 63.05 3 21b 70 +++ + ++ 4.61 77.77 4 35b 69 +++ NS NS 4.05 76.14 5 38 77 ++ + ++ 3.20 72.63 6 44 92 +++ + NS 2.42 67.59 7 47 84 ++ +++ ++ 2.22 65.76 0.98 s 48~ 66 + + + 44.46 9 53 88 + + NS 2.41 67.43 10 54 106 ++ + ++ 3.38 73.52 11 55 90 + +++ ++ 1.26 51 .65 12 58 a 67 +++ ++ ++ 13.21 85.67 13 73 109 +++ +++ +++ 4.77 78.15 14 80 87 + ++ +++ 2.57 68.74 15 82a 68 + + + 2.66 69.40 16 84a 65 - - -5.57 -79.81 17 90 82 . - -3.88 -75.54 18 96a 69 - . - -3.73 -74.99 19 98 85 . - - -2.74 -69.92 20 127 98 + NS NS 2.18 65.32

113 Note: *) design 1-day rainfall for 20 years return period, Notog, while for other water gauge there was a NS means no significant tendency, +++ indicate decreasing tendency. An example of the increasing significant increasing tendency with 95% confidence trend at Comal-Kecepit can be found in Fig. 3. level, -H- indicate moderate increasing tendency with 90 Pielke and Downton (1999) discussed about % confidence level, + indicate mild increasing tendency the relationship between the tendency of the with 85 % confidence level, --- indicate significant decreasing tendency with 95% confidence level, -­ extreme rainfall and flood discharge in the United indicate moderate decreasing tendency with 90 % States. The result of the study indicates that "the confidence level, - indicate mild decreasing tendency increase in precipitation is insufficient to explain with 85 % confidence level. increasing flood damages or variability in flood

Figure 1. Tendencies of extreme rainfall and annual maximum flood. The sign of significant trend is indicated by+ I- symbols at each site; area with grid indicates a positive tendency and area with horizontal line indicates negative tendency

400 -· .------~ 350 - 300 ..• ....-~•, . . I 250 '"• -.: :::· .. •'' ~ 200 .... • .. '* •' X~ ,.. s_ ol I .. tl JC ' I ,.t c "' ! 'ti~ h: 41! 11~: I'' ~ ~ · · •.a.Jtl: • •• ii' ,J .. .. :; • 150 .•jr11 ... , .,. • • •• 100 ~· ~· .: • 50

0 -· ~----r-----.-----r----,r----,r----,----~ 1880 1900 1920 1940 1960 1980 2000 2020 Year

Figure 2. Average intensity (solid line) and trend line with linier regression (dash line) of annual maximum daily rainfall of station 73.

Trend analysis for the data series of annual damages. " The study strongly suggests that the maximum discharge has a difficulty because of the societal factors-growth in population and wealth are short of data. From the five gauge stations, there is partly responsible for the observed trend in flood, no trend indication, mild, moderate, or strong damages. It means that the increase of rainfall tendency. Using a linear regression, it only intensity is not the main reason for the increase of indicates an increase or decrease from the normal the flood hazard, because other factors are more average. An increasing trend can be found in two significant. Conversely, the absent of increasing water gauges, i.e. Comal-Kecepit and Pemali 100 90 - )K ~ 80 - u ~ Cl) 70 - ::t: . :.::. -· I ~~- ~ ~JK ~ :.::, - .~ ' •:.:: r .. C'? !. •• ! 'f ·:.:: ·. ; g 60 - ."i !i: 50 .. ... ' ~.i :.:: :.:: ~ 40 !u J! 30 - c 20 10 0 1970 1975 1980 y~~~5 1990 1995 2000 2005

Figure 3. Average intensity (solid line) and trend line with linier regression (dash line) of annual maximum flood of Comal-Kecepit

flood \ndication does not mean neglecting the probability of detecting trends in measured stream increase of the rainfall intensity. flow may be very low. Relationship of tendency between rainfall Note: NS: No significant tendency and flood discharge is not easy to conduct, because of the short of Uood discharge data, but the CONCLUSION tendency basically can be seen from the influence Observation to the 1-day, 2-day, and 3-day of the location of rainfall stations to the location of maximum annual rainf:all from 57 rainfall stations water gauge. Table 2 demonstrates that not all with more than 50 years of record included the last rainfall tendency causes similar flood discharge tendency. It can be understood, because the two decades data in the Pekalongan region indicate available discharge data is not sufficient, but the that only 10% has an increasing tendency, and only 1% has a decreasing tendency. For the 2-day and direction of the rainfall tendency and flood 3-day maximum annual rainfall which has a discharge was similar. If there is a longer discharge positive tendency are even with a smaller data, there is a possibility that it will have a strong tendency as the rainfall, or even stronger due to the percentage. There are only two stations whic~1 bas .land use changes. a positive tendency for the three events With a length of data of 71 years and 109 years. ':"ith a Tlte analysis in this study was limited to small percentage of tendency, it cannot be said that point rainfall. With spatiaJ influence from other extreme rainfall has a change during the last two stations with no indication of increasing or decades. If the upper outlier in the data series was decreasing tendency, then rainfall stations whi~h have a tendency influence to discharge changes wtll excluded, it has only a small influ.ence to the level be eliminated. In Qther word, the existence or the of tendency, except for the decreasmg tendency .. absence of changes in flood discharge is not Design rainfall of 20-year return penod using the non-stationer data (with a tendency) for n~essarily influenced by the changes of rainfall. The influence of flood frequency in a peak over every decade demonstrates a sign~ficant change, thresh hold might be better to indicate the changes. either decreasing or increasing. This phenomenon McCabe and Wolock (1997) stated that the needs further consideration because the data series detection of trend for the flood discharge is more are stationer, whereas the data used for the rainfall or discharge estimation was usually assumed of difficult compared to rainfall, even though in fact the data series has a trend. The reason of this is the non-stationer. Maximum annual flood analysis does not Table 2. The relationship between extreme rainfall and flood tendencies.

No. AWLR Changing from the Rainfall Station Tendency avera1re 1 Pemali - Notog + 48a, 53, 54 +,+,++ 2 Comal-Kecepit + 73 +H- 3 Comal-Watukumpul - 90 --- 4 Kupang- Pagerukir - 128, 128a, 139, 139a NS 5 DonowiU'ih - 50, 59, 61a,60, 62 NS

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11 ~