Spatio-Temporal Analysis of Precipitation Frequency in Texas Using High-Resolution Radar Products

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Article Spatio-Temporal Analysis of Precipitation Frequency in Texas Using High-Resolution Radar Products

Dawit Ghebreyesus * and Hatim O. Sharif Department of Civil and Environmental Engineering, University of Texas at San Antonio, San Antonio, TX 78249, USA; [email protected] * Correspondence: [email protected]

Received: 13 April 2020; Accepted: 9 May 2020; Published: 13 May 2020

Abstract: Understanding the frequency and intensity of precipitation is needed for many vital applications including water supply for agricultural, municipal, industrial, and power generation uses, design of hydraulic structures, and analysis and forecasting of hazards such as ﬂood, drought, and landslide. This study examines, in detail, the spatial and temporal variability of precipitation frequency over the State of Texas and its trends from 2002 to 2019. The results indicate that Texas receives around 325 wet hours on average annually (3.7% of the time). The northern part of the Gulf Coast region witnesses the highest average precipitation frequency reaching 876 wet hours annually. The year 2015 was found to have the highest precipitation frequency across the state with an average frequency of 6% (525 wet hours) and 2011 was the driest, with an average frequency of 1.9% (170 wet hours). In terms of seasonality, the highest precipitation frequency was observed in the summer with a frequency of 4.1%. The areal average time-series of the precipitation frequency indicates that the 2011–2012 drought to be a change point. The Mann–Kendall trend analysis shows that 16.2% of the state experienced a signiﬁcant positive trend in precipitation frequency including the dry western region and major cities. The results can provide useful information about storm characteristics and recent change and variability of precipitation at high spatial resolutions and can be used in a multitude of practical applications.

Keywords: NEXt-generation Weather RADar (NEXRAD); precipitation; Texas; radar; STAGE IV

1. Introduction Precipitation is an essential input for water resource management studies and modeling of hydrological processes. In addition to being primary variables in climate classification, the frequency and intensity of precipitation are necessary for many vital applications including water supply for agricultural, municipal, industrial, and power-generation uses, design of hydraulic structures, and flood, drought, and landslide analysis and forecasting. However, an accurate representation of the spatial and temporal distribution of precipitation remains a major challenge facing the scientific community. Rain gauges provide accurate point measurements of precipitation, but their measurements are typically not enough to capture its spatial variability. Yatagai [1] showed that the use of spatially interpolated data obtained from a dense network of rain gauges significantly improved the spatial representation of the precipitation, but such resolutions are rarely available from existing rain gauge networks. Accordingly, numerous studies suggested that it is more efficient to use remotely sensed precipitation estimation techniques (radar-based and satellite-based) to capture the spatio-temporal variability of the precipitation [2–8]. The NEXt-generation Weather RADar (NEXRAD) systems have revolutionized the National Weather Service (NWS) forecast and warning programs by enhancing the detection mechanism of the severe weather such as wind, heavy rainfall, hail, hurricanes and tornadoes [9,10]. The network

Water 2020, 12, 1378; doi:10.3390/w12051378 www.mdpi.com/journal/water Water 2020, 12, 1378 2 of 16 consists of 160 WSR-88D (Weather Surveillance Radar-1988 Doppler) radars distributed across the entire conterminous United States. The National Centers for Environmental Prediction (NCEP) stage IV quantitative precipitation estimates (QPEs) are the most widely used NEXRAD precipitation products by the research community [6,11]. Examples of the studies conducted using the products include hydrologic model evaluation [12], flood forecasting [13], radar QPE evaluation and comparison with ground measurements [2], and assessment of different satellite precipitation products [14]. Gourley [15] used both NCEP Stage IV and Satellite QPEs to force a hydrological model and compare their performance against observed discharge, citing encouraging results. Yilmaz [12] compared gauge-based, radar-based, and satellite-based precipitation estimates accounting for hydrologic modeling and highlighted the benefits of spatial coverage by the radar product. Furl [6] assessed several satellite-based precipitation products versus the NCEP stage IV after deciding that the radar product was superior even to the sparse rain gauge estimates. Several studies were performed to compare the stage IV product with rain gauges. Wang’s [2] findings suggested that NEXRAD Multi-sensor Precipitation Estimator (MPE) has a much higher correlation than the previous products and underestimates by only 7% on average in a study conducted over the Upper Guadalupe River in Texas. The study concluded that the spatial variability of the precipitation was captured at a reasonable accuracy by the NEXRAD MPE. Westcott [16] compared monthly gauges amounts to that of the stage IV product over the Midwest and found that stage IV overestimated precipitation at the low end and underestimated it for high values. Even though the comparison was carried on the averaged values at a county level, MPE accuracy increased as the value of precipitation increases, except for rare heavy events. A similar conclusion was reached by Habib [17] in a validation study using a dense network of gauges carried over the south of Louisiana. Habib [17] further emphasized that the MPE Stage IV algorithm showed a significant improvement in its performance compared to its predecessors. All of the aforementioned studies did not claim that the Stage IV product was as accurate as rain gauge but suggested that it was capable of providing a very reasonable spatial representation of the precipitation with acceptable accuracy. Radar products were highly recommended for modeling practices that require high accuracy in the spatial and temporal distribution of the precipitation. In recent decades, numerous studies reported significant changes in mean precipitation over many regions not only in the US but throughout the world [18]. Several studies reported changes in the frequency and intensity of extreme events, i.e., floods and drought. Heavy precipitation events have become more frequent since the middle of the last century as reported by Osborn [19] in the UK, Sen Roy [20] in India, and by Karl [21] in the US. Precipitation has increased substantially in the state of Texas for the months December to March with an increase as large as 47% per century in Panhandle and Plains [22]. The steadiest increase, with a 15% per century rise, has been seen in East Texas, which receives a greater amount of precipitation during December to March than any other Texas climatic regions, in both absolute and relative terms. Overall, long-term precipitation trends range from about 5% per century in the Panhandle and Plains and Far West Texas to about 20% per century in South Texas and Southeast Texas. Drought has persisted in the TexMex Region, including Eastern Texas in 2011 and 2012 [23] but was followed by very wet years starting from 2015. The cost of the 2011 drought in Texas and surrounding regions in terms of U.S. agricultural losses was estimated to be $12 billion by The National Climatic Data Center [24]. Even though the most significant attribute of precipitation is the amount of precipitation that most of the rain gauges capture daily, precipitation frequency is another crucial component that needs in-depth analysis. The frequency of precipitation reveals information that is crucial in the water resource management and agricultural sectors, e.g., for better estimation of the amount and timing of irrigation and fertilizer application [25]. Only a few studies examined the precipitation frequency on regional and global scales [25,26]. The main limitation of these studies was that the majority were conducted at coarse spatial and temporal resolutions and, therefore, mask a significant part of the inherent variability of precipitation. For example, a daily analysis will provide very limited information Water 2020, 12, 1378 3 of 16

Waterbecause 2020 it, 12 seldom, x FOR PEER rains REVIEW all day. Moreover, the spatial resolution used in these studies (0.5 0.53 ofand 17 ◦ × ◦ 2.5 2.5 ) is only suitable for synoptic analysis at a global or continental scale. This study investigates ◦ × ◦ resolutionsthe precipitation from the frequency last 18 acrossyears using the state NEXRAD of Texas data with at sub-daily4 × 4 km2 temporalspatial resolution resolutions and from hourly the temporallast 18 years resolution. using NEXRAD The analysis data provides at 4 4 details km2 spatial of the spatial resolution distribution and hourly of the temporal change in resolution. trend of × theThe precipitation analysis provides frequency. details Moreover, of the spatial the distributionseasonal variability of the change in the in hourly trend offrequency the precipitation and the variabilityfrequency. of Moreover, frequency the versus seasonal the variabilityprecipitation in theintensity hourly will frequency be investigated. and the variability Precipitation of frequency of urban centersversus theis analyzed precipitation in detail intensity with the will focus be investigated. of the biggest Precipitation metropolitan of regions urban centersin the state is analyzed of Texas. in detail with the focus of the biggest metropolitan regions in the state of Texas. 2. Study Area 2. Study Area The State of Texas climate varies widely from arid in the west to very humid in the east. AccordingThe State to the of TexasKoppen–Geigre climate varies climate widely classifica fromtion, arid 63% in the of westthe state to very is regarded humid in as the warm east. temperate,According tofully the humid Koppen–Geigre with a hot climate summer. classification, The wester 63%n region of the is statedivided is regarded into four as climate warm categories temperate, (Figurefully humid 1). The with temperature a hot summer. greatly The varies western spatially region across is divided the state into fourand it climate can reach categories as high (Figure as 45 °C1). inThe the temperature summer (Jun–August) greatly varies spatiallyand well across below the the state freezing and it can point reach in as winter. high as Precipitation 45 ◦C in the summer varies significantly(Jun–August) across and well the belowstate with the the freezing eastern point part in (east winter. of I-35), Precipitation which receives varies significantlyover 800 mm across annually the andstate the with western the eastern part receiving part (east as of little I-35), as which 200 mm receives annually. over 800The mm State annually of Texas and is a the host western of several part majorreceiving rivers as littlesuch as as 200 the mm Rio annually.Grande River, The State Colorado of Texas River, is a host and of Guadalupe several major River. rivers Almost such asall the of the Rio riversGrande flow River, in a Colorado direction River, from andnorthwest Guadalupe to southeast, River. Almost eventually all of emptying the rivers into flow the in aGulf direction of Mexico. from Thenorthwest state is to exposed southeast, to frequent eventually extreme emptying weather into ev theents Gulf including of Mexico. major The tropical state is storms exposed and to drought frequent spells.extreme Moreover, weather events Texas includingis ranked majorfirst in tropical the US stormsfor tornados, and drought with an spells. average Moreover, annual Texas occurrence is ranked of 155first tornadoes in the US for[27]. tornados, with an average annual occurrence of 155 tornadoes [27].

FigureFigure 1. 1. TexasTexas climate climate according according to the Kopp Koppen–Geigreen–Geigre Climate Classification. Classiﬁcation.

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3. Methodology and Dataset The radar-based precipitation product, NWS/NCEP stage IV QPEs is used in this study. The NCEP stage IV product, herein referred to as NEXRAD stage IV, is a near-real-time product that is generated at NCEP separately based on NEXRAD Precipitation Processing System (PPS) [28] and compiled by the NWS 12 River Forecast Center (RFC) [29]. The main goal of the product was to provide quantitative precipitation forecasts (QPFs) by being used as a driving force into atmospheric forecast models [30]. Currently, the product is used for many applications and is a highly recommended precipitation product for the conterminous US, especially for moderate to heavy precipitation events. However, the product has some discontinuities due to operational processing at the radar site as well as discontinuities due to the merging of data from different River Forecast Centers (RFCs) [14]. This problem does not affect this analysis since almost all of the state of Texas (area of interest) falls within one River Forecast center, the West Gulf RFC. The main advantage of the NEXRAD Stage IV product is that the bias is adjusted in near-real time as compared to a lengthy delay of adjustment for the satellite-based gridded products available. The NEXRAD Stage IV data have an hourly temporal resolution, integrated from the 5–6 min native products, and a spatial resolution of 4 km covering the entire conterminous United States. For this study, a time frame covering a period of 18 years, starting from 2002 to 2019 is used. The data were obtained from the National Center for Atmospheric Research (NCAR) FTP servers as a GRIB (GRIdded Binary or General Regularly distributed Information in Binary) format. More detailed information about the NEXRAD stage IV data can be obtained from their website. The main limitation of the study is the relatively short data record used. The discontinuity in the data due to the operational processing at the radar site may have slightly affected the results, especially in the western region. The frequencies described are contingent upon the radar precipitation threshold used and the NEXRAD products’ accuracy. The analysis followed a path-flow starting from data acquisition of the NEXRAD stage IV dataset. The data contains some missing data throughout the study period. The missing data are ignored as there are about only 6 missing hourly files in a year on average, which is less than 0.07% of the annual data. It is reasonable to assume that the impact of the missing files on the frequencies to be insignificant. Pixel-wise data analysis was carried to calculate the precipitation frequency of the State of Texas. The analysis included generating precipitation frequency over the state at different time scales. Furthermore, the precipitation frequency was categorized into three groups according to the intensity of the precipitation. The threshold intensity of the groups was obtained from the rainfall categories set by the American Meteorological Society. Light intensity was set for rainfall events ranging from 0.02 mm h 1 to 2.5 mm h 1. The moderate rainfall intensity was set for rainfall events · − · − between 2.5 mm h 1 to 7.6 mm h 1 and the heavy event was set for events higher than 7.6 mm h 1. · − · − · − The four seasons were divided according to the Equinox and Solstice obtained from the NWS as follows: Spring (20 March to 20 June), Summer (21 June to 22 September), Autumn (23 September to 20 December), and Winter (21 December to 19 March). The precipitation frequency analysis was conducted for each of the four seasons. Trend analysis was further carried on the annual precipitation frequency to capture the evolution and spatial distribution of the significant trend if available. The Mann–Kendall test—one of the best non-parametric trend analysis tools for data with unknown distribution was used to assess the trends of the precipitation frequency. The non-parametric test of Mann–Kendall [31,32] was applied to the time-series of the annual precipitation frequency to assess the significance of the trend. The data sample was arranged on a yearly basis from 2002 to 2019 as x2002, x2003, x2004, ... , x2019 where x is the annual average hourly precipitation frequency. The Mann–Kendall test statics S is defined by:

n 1 n X− X S = sgn x x (1) j − i i=1 j=i+1 Water 2020, 12, 1378 5 of 16

where xi and xj are annual average precipitation values for the period of i and j, respectively, and the sgn(x x ) is given by the following signum function in Equation (2): j− i   1 if xj xi > 0  − sgn xj xi =  0 if xj xi = 0 (2) −  −  1 if x x < 0 − j − i For n greater than 10, the distribution of the S statistic can be approximated by a normal distribution with mean and variance given, respectively, in Equations (3) and (4) below:

E(S) = 0 (3)

Pg n(n 1)(2n + 5) = ti(i)(i 1)(2i + 5) Var(S) = − − i 1 − (4) 18 where ‘g’ is the number of tied groups and ‘ti’ is the number of observations in the ith group. For example, if the sequence of the precipitation frequencies in time from 2002 to 2019 is given in order {6, 3, 5, 3, 4, 1, 8, 3, 4, 9, 7, 7, 10, 11, 6, 4, 3, 15}. The n of the set will be 18, g will be equal to 4 because we have four tied groups {6,3,4,7}. The t1 = 2 for the value of 6 (6 is found twice), t2 = 4 for the value of 3, t3 = 3 for the value of 4 and t4 = 2 for the value of 7. The VAR(S) will be equal to 664.67 for the example set above. Then the Mann–Kendall test statistic will be calculated as shown in Equation (5):

 S 1  − if S > 0  √VAR(S)  ZMK =  0 if S = 0 (5)   S+1 if S < 0  √VAR(S)

A positive value of ZMK indicates an increasing trend, while a negative value of ZMK indicates a decreasing trend. The null hypothesis can be rejected at a significance level of p if |Zs| is greater than Z1 p/2, where Z1 p/2 can be obtained from the standard normal cumulative distribution tables. In the − − above example where S and VAR(S) are given by 44 and 664.67, respectively, the ZMK will be 1.67. The Z1 0.05/2 statistic, which is the Z-score of at significance level of 5% is 2.0 for the normal distribution − tables. Since ZMK is not greater than Z0.975, one can not reject the null hypothesis, which states that there is no monotonic trend, implying no significant trend. For the annual precipitation frequency, a significance level of 5% is adopted. The trend rate was then estimated for the radar pixels with a significant trend from the Mann–Kendall test. Equation (6) was used to estimate the rate of the trend over the decade as follows: Pn i=1 xiyi nxy Rate = P − 10 (6) n x 2 nx × i=1 i − where xi are the years; yi are the annual precipitation frequencies; Rate is in percentage point in a decade

4. Results and Discussion

4.1. Average Annual Frequency The average frequency of daily precipitation over the state of Texas is 22.01% (the average annual percentage of days that the radar reported as a precipitation day), as shown in Figure2. The daily frequency ranges from as low as 9% in the Western region to as high as 41% in the Gulf region. This means that Texas has, on average, 80 rainy days annually. The driest areas in the state witnessed 30 rainy days and the wettest area get on average more than 150 rainy days annually, according to the Stage IV data from 2002 to 2019. This is consistent with National Oceanic and Atmospheric Administration’s (NOAA) Climate Normals (1981–2010) for the western region. For example, according to NOAA, Water 2020, 12, 1378 6 of 16

Lajitas (located in the western part) receives 36 rainy days on average. According to the STAGE IV data, the place received 39 rainy days on average from 2002–2019, which is not significantly different. However, for the wettest regions in the eastern part of the state, there is a significant deviation from the Climate Normals. For example, Houston and Beaumont recorded 104 and 114 rainy days in NOAA

ClimateWater 2020 Normals;, 12, x FOR PEER however, REVIEW according to the Stage IV data, they showed a much higher number6 of of17 rainy days, 144 and 128, respectively. This is mostly due to the abnormally wet years after 2012 in this partfrequency of the state.cannot Moreover, provide in-depth this daily information analysis of theabout precipitation the frequency frequency because cannot it treats provide 5 min in-depthrain and information8 h rain the aboutsame theas a frequency rainy day. because That is it why treats it 5is min crucial rain to and capture 8 h rain the the sub-daily same as precipitation a rainy day. Thatfrequency. is why it is crucial to capture the sub-daily precipitation frequency.

FigureFigure 2.2. Spatial distribution of daily average precipitationprecipitation frequencyfrequency inin thethe StateState ofof Texas.Texas.

TheThe average annual frequency frequency of of hourly hourly precipitat precipitationion (percentage (percentage of ofthe the time time it rains) it rains) over over the thestudy study period period across across the theState State of Texas of Texas has hasa spatial a spatial variability variability ranging ranging from from almost almost 1% 1%to 9%. to 9%.As Asexpected, expected, the theeastern eastern part part of the of thestate, state, east east of th ofe theI-35, I-35, shows shows higher higher frequency frequency values values that thatare well are wellabove above 5% in 5% general. in general. The The area area with with the the highest highest precipitation precipitation frequency frequency is is east east of of the HoustonHouston MetropolitanMetropolitan area.area. OnlyOnly 0.08%0.08% (~560(~560 km2)) of of the the state state receive receive precipitation frequency more than 7%. AboutAbout 3%3% (~21,000(~21,000 km km22)) ofof the the state state of of Texas, Texas, most most of of which which is locatedis located in in the the western western region, region, shows shows an annualan annual average average frequency frequency of lessof less than than 2%. 2%. The The western western part part of of the the state state near near the the city city of of ElEl PasoPaso hashas registeredregistered thethe lowest lowest precipitation precipitation frequency. frequency. On On average, average, the the state state of Texasof Texas witnessed witnessed around around 325 wet325 hourswet hours annually annually (3.7%). (3.7%). Some Some of the of wettest the wettest areas area receiveds received as many as many as 876 as wet 876 hours wet hours on average on average in the lastin the 18 years.last 18 Theyears. spatial The distributionspatial distribution of the annual of the average annual hourlyaverage precipitation hourly precipitation frequency frequency is shown inis Figureshown3 A.in TheFigure spatial 3A. distributionThe spatial ofdistribution the average of annual the average cumulative annual rainfall cumulative follows rainfall a similar follows pattern a assimilar the hourly pattern precipitation as the hourly frequency. precipitation Average frequency. annual Average rainfall in annual the state rainfall of Texas, in the according state of Texas, to the Stageaccording IV data, to the was Stage found IV data, to be was 720 found mm over to be the 720 study mm over period. the study Again, period. the region Again, with the theregion highest with averagethe highest annual average cumulative annual rainfall cumulative is located rainfall east ofis thelocated Houston east Metropolitanof the Houston area Metropolitan (Easter tip of area the I-10)(Easter with tip theof the observed I-10) with amount the observed of around amount 1860 mm.of around The driest 1860 mm. region The of driest the state region is located of the state in the is westernlocated in part the with western annual part average with annual precipitation average as precipitation low as 125 mm as low (Figure as 1253B). mm (Figure 3B).

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Water 2020, 12, 1378 7 of 16 Water 2020, 12, x FOR PEER REVIEW 7 of 17

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Figure 3. (A) Spatial distribution of the average annual frequency of hourly precipitation in the State ofFigure TexasFigure 3.and 3.(A ( A()B Spatial)) Spatial the annual distribution distribution average of of thecumulative the average average annualprecipitation. annual frequency frequency of of hourly hourly precipitation precipitation in in the the State State of Texasof Texas and (andB) the (B) annual the annual average average cumulative cumulative precipitation. precipitation. When separated according to the precipitation intensity, the frequency of light precipitation eventsWhen (<2.5When mm/h) separated separated follows according according the same toto thepatternthe precipitationprecipitation as the average intensity,intensity, shown thethe in frequencyfrequency Figure 3A. ofof The lightlight Northeastern precipitation regioneventsevents (East (< (<2.52.5 of mm mm/h) I-45)/h) follows showsfollows thehigh the same same light-event patternpattern asprecipitationas thethe averageaverage frequency, shownshown inin FigureFigurewith its3 3A.A. peak TheThe NortheasternlyingNortheastern in the Houstonregionregion (East Metropolitan(East of I-45)of I-45) shows area shows highand high light-eventits surrounding light-event precipitation precipitationareas. The frequency, average frequency, with light-event its with peak its frequency lying peak in lying the (2.6%) Houston in theis onlyMetropolitanHouston one percentage Metropolitan area andpoint its area lower surrounding and than its thesurrounding areas.average The an area averagenuals. precipitationThe light-event average light-eventfrequency. frequency Thisfrequency (2.6%) indicates is only(2.6%) that one is onpercentage onlyaverage one 77%percentage point of lower the precipitationpoint than lower the average than in Texa the annual averages is light-event, precipitation annual precipitationi.e., frequency. less than Thisfrequency. 2.5 mm·h indicates− This1. In that theindicates on case average of that a 1 moderate77%on ofaverage the event precipitation 77% (2.5 of mm/h–7.6 the precipitation in Texas mm/h), is light-event, inthe Texa coastals is i.e.,light-event,areas less between than i.e., 2.5 highway less mm thanh− I-10 .2.5 In mm·hand the caseI-30−1. Inand of the a bounded moderate case of a · byevent moderateI-45 (2.5are found mm event/h–7.6 to (2.5 the mm mm/h–7.6 region/h), the with coastalmm/h), the highest areasthe coastal between frequency. areas highway between The average I-10 highway and moderate-event I-30 I-10 and and bounded I-30 precipitation and by bounded I-45 are frequencyfoundby I-45 to are theacross found region Texas to with the 0.64%. region the highest Hence, with frequency.the moderate-event highest Thefrequency. average precipitation The moderate-event average covers moderate-event about precipitation 17% precipitationof frequencythe total precipitationacrossfrequency Texas inacross 0.64%. Texas. Texas Hence, The 0.64%. frequency-heavy moderate-event Hence, moderate-event precipitation precipitation coversprecipitation events about (>7.617% mm/h)covers ofthe aboutare total very 17% precipitation small of the across total in theTexas.precipitation state The except frequency-heavy forin Texas. a small The area frequency-heavy precipitation located east events from precip Houston (>7.6itation mm parallel events/h) are to(>7.6 very I-10, smallmm/h) which across are has very thea frequency small state exceptacross of 0.5%–0.76%.forthe a state small except The area heavy locatedfor a smallevents east area fromare locatedvery Houston rare, east in parallelfrom general, Houston to in I-10, the parallel state which of to hasTexas, I-10, a frequency whichcovering has only ofa frequency 0.5%–0.76%. 6% of the of totalThe0.5%–0.76%. rainfall. heavy events In The summary, areheavy very events the rare, Houston inare general, very metropolitan rare, in thein general, state area of inTexas, is the found state covering to of have Texas, only the covering 6%highest of the onlyfrequency total 6% rainfall. of thein termsIntotal summary, of rainfall. light, themoderate In Houston summary, and metropolitan heavythe Houston precipitation area metropolitan is found and is to areathe have wettest is thefound highest region to have frequencyin thethe highestentire in state terms frequency overall. of light, in Themoderateterms western of andlight, part heavy moderateof the precipitation state and is theheavy anddriest precipitation is and the wettesthas the and region lowest is the in frequencies wettest the entire region state for allin overall. thetypes entire Theof precipitation state western overall. part events.ofThe the westernstateThe spatial is the part driest distributionof the and state has is of the the the lowest driest light-, frequenciesand moderate-, has the forlowest and all types heavy-eventfrequencies of precipitation for frequencies all types events. of are precipitation Theshown spatial in Figuredistributionevents. 4. The of spatial the light-, distribution moderate-, of the and light-, heavy-event moderate-, frequencies and heavy-event are shown frequencies in Figure4 .are shown in Figure 4.

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Figure 4. Spatial distribution of the average annual frequency of hourly precipitation for different Figure 4. Spatial distribution of the average annual frequency of hourly precipitation for diﬀerent intensitiesFigure 4.(A Spatial) light; (distributionB) moderate of intensity; the average (C) heavy.annual frequency of hourly precipitation for different intensitiesintensities (A (A) light;) light; (B ()B moderate) moderate intensity; intensity; ( C(C)) heavy. heavy.

TheThe highest highest precipitation precipitation frequency frequency over over the the stud studyy period period was was observed observed in in 2015, 2015, which which was was also also The highest precipitation frequency over the study period was observed in 2015, which was also the wettest year in terms of the total accumulated precipitation (Figure 5). In that year, Texas showed thethe wettest wettest year year in in terms terms of of the the total total accumulated accumulated precipitationprecipitation (Figure(Figure5 5).). InIn thatthat year,year, TexasTexas showedshowed an average precipitation frequency of 6%, which is almost double the annual average. This means anan average average precipitation precipitation frequency frequency of 6%,of 6%, which which is almost is almost double double the annualthe annual average. average. This This means means vast vast areas of Texas experienced more than 525 h of precipitation in 2015. Almost 30% (~200,0002 km2) areasvast ofareas Texas of experiencedTexas experienced more than more 525 than h of 525 precipitation h of precipitation in 2015. in Almost 2015. Almost 30% (~200,000 30% (~200,000 km ) of km the2)

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Water 2020, 12, x FOR PEER REVIEW 8 of 17 stateWater experienced 2020, 12, x FOR a PEER precipitation REVIEW frequency of more than 7% (which is 99th percentile of the average8 of 17 of the state experienced a precipitation frequency of more than 7% (which is 99th percentile of the annual frequency). The lowest frequency of hourly precipitation occurred in 2011, which was also the average annual frequency). The lowest frequency of hourly precipitation occurred in 2011, which was driestof the year state over experienced the study a period precipitation when the frequency state experienced of more than a devastating 7% (which drought is 99th (Figurepercentile5). Inof thatthe also the driest year over the study period when the state experienced a devastating drought (Figure year,average the entireannual state frequency). experienced The lowest a precipitation frequency frequencyof hourly precipitation of less than 3%occurred with anin 2011, exception which of was the 5). In that year, the entire state experienced a precipitation frequency of less than 3% with an northeasternalso the driest tip. year This over means the study that theperiod majority when ofthe the state state experienced had less than a devastating 250 rainy drought hours that (Figure year. exception of the northeastern tip. This means that the majority of the state had less than 250 rainy The5). averageIn that precipitationyear, the entire frequency state experienced was only 1.9% a precipitation in 2011. In that frequency year, only of 4%less of than the state3% with received an hours that year. The average precipitation frequency was only 1.9% in 2011. In that year, only 4% of anexception average frequencyof the northeastern more than tip. the This state’s means average that annual the majority precipitation of the state frequency, had less the than remaining 250 rainy 96% the state received an average frequency more than the state’s average annual precipitation frequency, ofhours Texas that experienced year. The aaverage precipitation precipitation frequency frequency below thewas state’s only 1.9% annual in 2011. average. In that year, only 4% of thethe remainingstate received 96% an of average Texas experiencedfrequency more a precipitation than the state’s frequency average below annual the precipitation state’s annual frequency, average. the remaining 96% of Texas experienced a precipitation frequency below the state’s annual average.

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Figure 5. Spatial distribution of the annual frequency of hourly precipitation of the (A) driest (2011) Figure 5. Spatial distribution of the annual frequency of hourly precipitation of the (A) driest (2011) Figure 5. Spatial distribution of the annual frequency of hourly precipitation of the (A) driest (2011) andand (B (B) wettest) wettest (2015) (2015) year year in in Texas Texas over over the the study study period. period. and (B) wettest (2015) year in Texas over the study period. TheThe annual annual cumulative cumulative precipitation precipitation of of the the wettest wettest and and the the driest driest year year of of the the study study period period is is The annual cumulative precipitation of the wettest and the driest year of the study period is shownshown in in Figure Figure6. 6. The The average average of of the the wettest wettest year year (2015) (2015) was was around around 1125 1125 mm mm and and the the driest driest year year shown in Figure 6. The average of the wettest year (2015) was around 1125 mm and the driest year (2011)(2011) average average was was found found to to be be around around 350 350 mm. mm. As As shown shown in in Figure Figure6, 6, the the area area east east of of the the Houston Houston (2011) average was found to be around 350 mm. As shown in Figure 6, the area east of the Houston metropolitanmetropolitan area area along along the the I-10 I-10 highway highway received received more more than than 2000 2000 mm mm in in 2015. 2015. However, However, in in 2011, 2011, metropolitan area along the I-10 highway received more than 2000 mm in 2015. However, in 2011, thethe highest highest observed observed amounts amounts over over the entire the entire state werestate barely were abovebarely 1000 above mm, 1000 with mm, the 95th with percentile the 95th the highest observed amounts over the entire state were barely above 1000 mm, with the 95th ofpercentile 800 mm showingof 800 mm the showing severity the of the severity drought. of the drought. percentile of 800 mm showing the severity of the drought.

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Figure 6. The Spatial distribution of the annual cumulative precipitation of the (A) driest and (B) the Figure 6. The Spatial distribution of the annual cumulative precipitation of the (A) driest and (B) the Figurewettestyear 6. The in Spatial Texas distribution over the study of the period. annual cumulative precipitation of the (A) driest and (B) the wettestyear in Texas over the study period. wettestyear in Texas over the study period.

Annual variability ofof precipitationprecipitation frequency frequency over over Texas Texas shows shows that that the the frequency frequency increased increased in in thethe latter years of thethe study.study. SevenSeven ofof thethe toptop nine nine highest highest frequencies frequencies were were seen seen from from 2013 2013 to to 2019 2019 (Figure(Figure 7). In the yearsyears beforebefore thethe 20112011 drought,drought, the the frequency frequency was was almost almost stable, stable, ranging ranging between between 3% and 5% of the average annualannual precipitationprecipitation fr frequency.equency. The The year year 2011 2011 marked marked the the change change point point

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Annual variability of precipitation frequency over Texas shows that the frequency increased in the latter years of the study. Seven of the top nine highest frequencies were seen from 2013 to 2019 (Figure7). In the years before the 2011 drought, the frequency was almost stable, ranging between 3% Water 2020, 12, x FOR PEER REVIEW 9 of 17 and 5% of the average annual precipitation frequency. The year 2011 marked the change point and the andfrequency the frequency showed showed a remarkable a remarkable rise after rise 2011 after to 2011 2012, to ranging 2012, ranging between between 4% and 4% 6% and areal 6% average areal averageprecipitation precipitation frequency frequency (Figure7 ).(Figure The average 7). The frequencyaverage frequency before the before 2011 droughtthe 2011 wasdrought about was 3.4% about and 3.4%the average and the frequencyaverage frequency after jumped after one jumped percentage one percentage point to 4.3%, point excluding to 4.3%, excluding 2011. 2011.

FigureFigure 7. 7. AnnualAnnual distribution distribution of of the the average average annu annualal frequency frequency of of hourly hourly precipitation. precipitation.

4.2.4.2. Precipitation Precipitation Frequency Frequency in in Major Major Metropolitan Metropolitan Areas Areas AA closer closer look look at at the the three three major major metropolitan metropolitan areas areas is warranted is warranted since since they theyhost hostmore morethan 50% than of50% the oftotal the population total population of the state of the of Texas state ofand Texas precipitation and precipitation affects daily aﬀ lifeects and daily traffic life in and these tra ﬃareasc in (e.g.,these [33]). areas The (e.g., Houston [33]). The Metropolitan Houston Metropolitan area (Greater area Houston) (Greater is Houston) the wettest is the with wettest an average with an precipitationaverage precipitation frequency frequency of 5.94%, which of 5.94%, is equivalent which is equivalentto almost 520 to wet almost hours 520 every wet hoursyear. The every spatial year. distributionThe spatial distributionof the precipitation of the precipitation frequency frequencyaround Houston around is Houston uniform is with uniform minor with variations minor variations (Figure 8A).(Figure The8 A).Dallas–Fort The Dallas–Fort Worth Metroplex Worth Metroplex area encompasses area encompasses 13 counties 13 counties within withinTexas and Texas is andhome is hometo 7.5 millionto 7.5 million residents, residents, making making it the itmost the mostpopulous populous metropolitan metropolitan area areain the in South the South and andfourth fourth largest largest in thein theUSA. USA. This This metropolitan metropolitan area areareceives receives an average an average precipitation precipitation frequency frequency of 4.79% of 4.79%(~420 wet (~420 hours wet perhours year). per The year). northeastern The northeastern region regionof the metrople of the metroplexx experiences experiences a higher a higherprecipitation precipitation frequency frequency than thethan western the western region region (Figure (Figure 8B).8 TheB).The third third major major metropolitan metropolitan area area in inTexas Texas is is the the San San Antonio–New Antonio–New BraunfelsBraunfels Metropolitan Metropolitan area, area, which which encompasses encompasses eight eight counties counties in in Texas. Texas. This This is is the the driest driest among among the the threethree aforementioned aforementioned areas, areas, with with an an average average precipitation precipitation frequency frequency of of 3.84%. 3.84%. The The wettest wettest region region of of SanSan Antonio–NewAntonio–New BraunfelsBraunfels Metropolitan Metropolitan area area is around is around the New the Braunfels New Braunfels area, with area, a precipitation with a precipitationfrequency of frequency 4.5%, which of equates4.5%, which to around equates 400 to wet around hours 400 annually, wet hours probably annually, due toprobably the orographic due to thelifting orographic on the Balcones lifting on Escarpment the Balcones edge Escarpment (Figure8C). edge (Figure 8C).

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Water 2020, 12, x FOR PEER REVIEW 9 of 17

and the frequency showed a remarkable rise after 2011 to 2012, ranging between 4% and 6% areal average precipitation frequency (Figure 7). The average frequency before the 2011 drought was about 3.4% and the average frequency after jumped one percentage point to 4.3%, excluding 2011.

Figure 7. Annual distribution of the average annual frequency of hourly precipitation.

4.2. Precipitation Frequency in Major Metropolitan Areas A closer look at the three major metropolitan areas is warranted since they host more than 50% of the total population of the state of Texas and precipitation affects daily life and traffic in these areas (e.g., [33]). The Houston Metropolitan area (Greater Houston) is the wettest with an average precipitation frequency of 5.94%, which is equivalent to almost 520 wet hours every year. The spatial distribution of the precipitation frequency around Houston is uniform with minor variations (Figure 8A). The Dallas–Fort Worth Metroplex area encompasses 13 counties within Texas and is home to 7.5 million residents, making it the most populous metropolitan area in the South and fourth largest in the USA. This metropolitan area receives an average precipitation frequency of 4.79% (~420 wet hours per year). The northeastern region of the metroplex experiences a higher precipitation frequency than the western region (Figure 8B). The third major metropolitan area in Texas is the San Antonio–New Braunfels Metropolitan area, which encompasses eight counties in Texas. This is the driest among the three aforementioned areas, with an average precipitation frequency of 3.84%. The wettest region of San Antonio–New Braunfels Metropolitan area is around the New Braunfels area, with a precipitation frequency of 4.5%, which equates to around 400 wet hours annually, probably due to Water 2020, 12, 1378 10 of 16 the orographic lifting on the Balcones Escarpment edge (Figure 8C).

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Figure 8. Spatial distribution of the average annual frequency of hourly precipitation for the major metropolitan areas in Texas (A) Houston; (B) Dallas–Fort Worth; (C) San Antonio–New Braunfels.

4.3. Seasonal Variability of Annual Frequency The spatial variability of the frequency of precipitation across the state was also shown in the seasonal cycle. In spring (20 March–20 June), the northeastern area bounded by I-20 and I-35 shows a higher precipitation frequency, with about 6%. The Houston metropolitan area also shows a higher precipitation frequency, especially south of Houston along the I-45 highway (Figure9A). The north-central part of the state shows an average precipitation frequency of about 4%. The entire coastal area east of I-45 was found to be a region with relatively high precipitation frequency. The average precipitation frequency for spring is found to be 3.6%, which is close to the annual average. As a threshold for the extreme precipitation frequency 95th percentile (130 wet hours, which is equivalent to 6%) of all the seasons’ combined was chosen. In the spring, only 1.5% of the state received more than 130 wet hours on average out of the three months. Summer is generally the wettest season of the year in Texas. On average, the state shows a 4.1% precipitation frequency translated to about 90 wet hours over the three months. In the summer, more than two-thirds of Texas shows a precipitation frequency higher than the annual average (3.7%). Moreover, 5% of the state experienced more than 130 wet hours in summer, which is three times larger than in spring. In the summer, the region with the highest precipitation frequency is the northern part of the Texas Gulf coast (Figure9B). The Houston metropolitan area and its surrounding area showed a very high precipitation frequency of above 8%. In autumn, the state is divided into two regions, west of I-35, with a precipitation frequency of less than 3.6% (average frequency), and east of I-35, with a precipitation frequency between 3.7% and 8%. The hotspots in autumn are located along the state’s border with Louisiana (areas between I-10 and I-30, Figure9C). The average seasonal precipitation frequency is slightly lower than the annual average frequency in autumn, with 3.6%. Regarding winter, the state is divided into three vertical strips (Figure9D). The western strip (west of I-35), the largest strip covering 61%, contains precipitation frequencies below 3.6%. The middle strip has a precipitation frequency between 3.6% and 6%. The eastern strip (the border with Louisiana), which covers about 11% of the state, has the highest precipitation frequency for the winter, with a range between 6% and 8%. Generally, the southwestern part of Texas (West Texas Region) receives the lowest precipitation frequency in all seasons, with less than 2% seasonal precipitation frequency (which is less than 45 wet hours in each season). The area with the highest precipitation frequency is the Houston Metropolitan area and its surroundings in all seasons (around 130 wet hours in each season). The monthly distribution of the precipitation frequency indicates that September is the wettest month in the state of Texas, with an average frequency of 4.7% (34 wet hours). The driest month was found to be April, with a frequency of 3.2% (having 23 wet hours). The months from May to September are the wettest months, with a precipitation frequency greater than 3.7% (Figure 10). Although the spatial distribution of winter showed a higher precipitation frequency (Figure9 above), the average Water 2020, 12, x FOR PEER REVIEW 11 of 17

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FigureFigure 9. 9.Spatial Spatial distributiondistribution ofof thethe averageaverage annualannual frequencyfrequency of hourly precipitation for for the the four four seasonsseasons in in Texas Texas ( A(A)) Spring; Spring; ( B(B)) Summer; Summer; ((CC)) Autumn;Autumn; ((D) Winter (calculated from from Hourly Hourly NEXRAD NEXRAD StageStage IV IV data data 2002–2019). 2002–2019).

TheSummer diurnal is generally cycle of the the precipitationwettest season frequency of the year was in Texas. also further On average, investigated the state across shows the a 4.1% state. Theprecipitation cycle showed frequency a bimodal translated distribution, to about with 90 wet a relatively hours over small the three peak months. in the early In the hours summer, of the more day (3.7%)than two-thirds and the strongest of Texas peak shows occurring a precipitation in the late frequency afternoon (Figurehigher than11). Thethe hourlyannual variabilityaverage (3.7%). of the precipitationMoreover, 5% frequency of the state clearly experienced showed thatmore most than of 130 the wet rainy hours hours in occur summer, during which the afternoonis three times and eveninglarger than (Figure in 11spring.). The In wettest the summer, hours being the fromregion 3:00 with p.m. the to highest 5:00 p.m. precipitation Central Standard frequency Time is (CST), the withnorthern a frequency part of ofthe about Texas 4.3%. Gulf coast The driest (Figure periods 9B). The of Houston the day occur metropolitan during thearea late and night its surrounding and around midday,area showed as shown a very in high Figure precipitation 11. Interestingly, frequency the two of above peaks 8%. in the hourly frequency coincide with the daily traﬃc peaks, amplifying the impacts on daily traﬃc [33]. Water 2020, 12, x FOR PEER REVIEW 12 of 17

In autumn, the state is divided into two regions, west of I-35, with a precipitation frequency of less than 3.6% (average frequency), and east of I-35, with a precipitation frequency between 3.7% and 8%. The hotspots in autumn are located along the state’s border with Louisiana (areas between I-10 and I-30, Figure 9C). The average seasonal precipitation frequency is slightly lower than the annual average frequency in autumn, with 3.6%. Regarding winter, the state is divided into three vertical strips (Figure 9D). The western strip (west of I-35), the largest strip covering 61%, contains precipitation frequencies below 3.6%. The middle strip has a precipitation frequency between 3.6% and 6%. The eastern strip (the border with Louisiana), which covers about 11% of the state, has the highest precipitation frequency for the winter, with a range between 6% and 8%. Generally, the southwestern part of Texas (West Texas Region) receives the lowest precipitation frequency in all seasons, with less than 2% seasonal precipitation frequency (which is less than 45 wet hours in each season). The area with the highest precipitation frequency is the Houston Metropolitan area and its surroundings in all seasons (around 130 wet hours in each season). The monthly distribution of the precipitation frequency indicates that September is the wettest month in the state of Texas, with an average frequency of 4.7% (34 wet hours). The driest month was found to be April, with a frequency of 3.2% (having 23 wet hours). The months from May to September are the wettest months, with a precipitation frequency greater than 3.7% (Figure 10). Although the spatial distribution of winter showed a higher precipitation frequency (Figure 9 above), the average monthly precipitation frequency was found to be relatively low (Months of January– March in Figure 10). This suggests that, in winter, only the areas that border with Louisiana (covering 11%) receive a higher frequency, with most of the state (61% of the state) receiving below the average precipitationWater 2020, 12, 1378 frequency. The summer months were also found to be amongst the wettest months12 of of 16 the year, consistent with the observations from the spatial analysis above.

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midday, as shown in Figure 11. Interestingly, the two peaks in the hourly frequency coincide with the daily trafficFigureFigure peaks, 10. 10. MonthlyMonthly amplifying distribution distribution the impacts of of the the average average on daily annu annual traffical frequency frequency [33]. of of hourly hourly precipitation. precipitation.

The diurnal cycle of the precipitation frequency was also further investigated across the state. The cycle showed a bimodal distribution, with a relatively small peak in the early hours of the day (3.7%) and the strongest peak occurring in the late afternoon (Figure 11). The hourly variability of the precipitation frequency clearly showed that most of the rainy hours occur during the afternoon and evening (Figure 11). The wettest hours being from 3:00 p.m. to 5:00 p.m. Central Standard Time (CST), with a frequency of about 4.3%. The driest periods of the day occur during the late night and around

FigureFigure 11. DiurnalDiurnal distribution distribution of of the the average average precipitation precipitation frequency.

4.4. Change Change in in Precipitation Precipitation Frequency over the Study Period Previous studies studies examined examined trends trends in inaverage average and and annual annual maximum maximum precipitation precipitation in Texas in Texas(e.g., [22,23,34]).(e.g., [22,23 ,Here,34]). Here,we perform we perform spatial spatial trend trendanalysis analysis of the ofannual the annual precipitation precipitation frequency. frequency. This analysisThis analysis will provide will provide a clear a picture clear picture of how of the how frequency the frequency of precipitation of precipitation across acrossthe state the of state Texas of has been changing over the study period. The Mann–Kendall trend test was used because the test is non-parametric and is not sensitive to the magnitude of precipitation frequency. A statistically significant positive trend in precipitation frequency is concentrated over in the western part of the State (Figure 12A). Most of the statistically significant trend in precipitation frequency is observed in a strip that runs from southwest to northeast, in addition to scattered spots around some major cities such as Austin, Houston, and Beaumont. The significance of the trend was analyzed using the Mann– Kendall method, with a significance level of 0.05. Significant positive trends in the precipitation frequency were observed in 16.1% (~105,000 km2) of the state, in which the majority of them are located in the driest region of the state. The rest of the state shows a statistically insignificant positive trend, with an approximately 9% insignificant negative trend. Out of these areas that showed a significant positive trend, more than 90% (~96,500 km2) were increasing at a rate of more than 1 percentage point per decade (Figure 12B).

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Texas has been changing over the study period. The Mann–Kendall trend test was used because the test is non-parametric and is not sensitive to the magnitude of precipitation frequency. A statistically significant positive trend in precipitation frequency is concentrated over in the western part of the State (Figure 12A). Most of the statistically significant trend in precipitation frequency is observed in a strip that runs from southwest to northeast, in addition to scattered spots around some major cities such as Austin, Houston, and Beaumont. The significance of the trend was analyzed using the Mann–Kendall method, with a significance level of 0.05. Significant positive trends in the precipitation frequency were observed in 16.1% (~105,000 km2) of the state, in which the majority of them are located in the driest region of the state. The rest of the state shows a statistically insignificant positive trend, with an approximately 9% insignificant negative trend. Out of these areas that showed a significant positive trend, more than 90% (~96,500 km2) were increasing at a rate of more than 1 percentage point per Water 2020, 12, x FOR PEER REVIEW 14 of 17 decade (Figure 12B).

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FigureFigure 12. 12.Spatial Spatial distribution distribution of of the the results results from from the the Mann–Kendall’s Mann–Kendall’s trend trend analysis analysis (A (A) Areas) Areas with with a significanta significant trend trend (red); (red); (B )(B the) the magnitude magnitude of of the the statistically statistically significant significant trendtrend raterate (percentage(percentage point perper decade). decade). 5. Conclusions 5. Conclusions Being the main driver of the hydrological cycle, accurately understanding the spatial and temporal Being the main driver of the hydrological cycle, accurately understanding the spatial and distribution of precipitation is very important for hydrologic modeling and forecasting and many temporal distribution of precipitation is very important for hydrologic modeling and forecasting and other applications. The frequency of precipitation (how often it rains) affects daily life in many ways many other applications. The frequency of precipitation (how often it rains) affects daily life in many and is very important for numerous water-resource applications. This study employs the spatially ways and is very important for numerous water-resource applications. This study employs the and temporally fine resolutions of precipitation, made possible by NEXRAD products, to analyze the spatially and temporally fine resolutions of precipitation, made possible by NEXRAD products, to variability of precipitation frequency over the state of Texas for the 2002–2019 period. We showed how analyze the variability of precipitation frequency over the state of Texas for the 2002–2019 period. We precipitation frequency can vary substantially across the state and at different time scales. The results showed how precipitation frequency can vary substantially across the state and at different time can help validate or invalidate some assumptions that are used in practice regarding the statistical scales. The results can help validate or invalidate some assumptions that are used in practice properties of storms (e.g., design storm frequency and area reduction factors). regarding the statistical properties of storms (e.g., design storm frequency and area reduction factors). TheThe analysis analysis was was conducted conducted using using NEXRAD NEXRAD StageStage IVIV data,data, radar-basedradar-based precipitationprecipitation product developeddeveloped by Nationalby National Weather Weather Service /NationalService/Nati Centersonal forCenters Environmental for Environmental Prediction (NWS Prediction/NCEP), over(NWS/NCEP), the state of Texas.over the Eighteen- state of Texas. years’ worthEighteen- of data year ats’ hourlyworth of resolution data at hour spanningly resolution the periods spanning from 2002the toperiods 2019 were from used 2002 to to investigate 2019 were the used spatial to investigate distribution the of spatial the precipitation distribution frequency. of the precipitation The spatial resolutionfrequency. of The the spatial data is resolution approximately of the data 4 km is byapproximately 4 km. The seasonal4 km by 4 featureskm. The ofseasonal the precipitation features of frequencythe precipitation were also frequency analyzed. were Furthermore, also analyzed. trend analysisFurthermore, of the trend precipitation analysisfrequency of the precipitation at the grid levelfrequency was conducted at the grid using level the was non-parametric conducted using Mann–Kendall the non-parametric test. Mann–Kendall test. Generally,Generally, the the eastern eastern region region of of the the state state was was found found toto havehave thethe highesthighest precipitation frequency andand the the western western region region the the lowest lowest (the (the frequency frequency of of precipitation precipitation increases increases from from west west to eastto east across across the the state). On average, the state receives around 325 wet hours annually, which translates to a frequency of 3.7%. The wettest region (south of the Houston metropolitan area) receives about 876 wet hours annually on average. More than three-quarters of the rain in the State of Texas happens to be a light event (less than 2.5 mm·h−1). As expected, most of the heavy events (larger than 7.6 mm·h−1) occurred in the eastern region, especially in the southern part of the border with the state of Louisiana. Annual variability of the frequency of the precipitation over Texas shows that seven of the nine years with the highest frequencies were seen after the devastating 2011–2012 drought. In the years before the 2011 drought, the frequency was almost stable, ranging between 3% and 3.5%, with the exceptions of two years. The rapid change started after the year 2011. The wettest year in terms of the precipitation frequency was 2015, with an average frequency of 6% over the entire state and 2011 was the driest, with an average frequency of 1.9% (~170 wet hours in the year 2011). Out of the three major metropolitan areas of the state, Houston was found to witness the highest precipitation frequency, with an average of about 520 wet hours per year, followed by Dallas-Fort

Water 2020, 12, 1378 14 of 16 state). On average, the state receives around 325 wet hours annually, which translates to a frequency of 3.7%. The wettest region (south of the Houston metropolitan area) receives about 876 wet hours annually on average. More than three-quarters of the rain in the State of Texas happens to be a light event (less than 2.5 mm h 1). As expected, most of the heavy events (larger than 7.6 mm h 1) occurred · − · − in the eastern region, especially in the southern part of the border with the state of Louisiana. Annual variability of the frequency of the precipitation over Texas shows that seven of the nine years with the highest frequencies were seen after the devastating 2011–2012 drought. In the years before the 2011 drought, the frequency was almost stable, ranging between 3% and 3.5%, with the exceptions of two years. The rapid change started after the year 2011. The wettest year in terms of the precipitation frequency was 2015, with an average frequency of 6% over the entire state and 2011 was the driest, with an average frequency of 1.9% (~170 wet hours in the year 2011). Out of the three major metropolitan areas of the state, Houston was found to witness the highest precipitation frequency, with an average of about 520 wet hours per year, followed by Dallas-Fort Worth (420) and San Antonio (400). The results of the daily precipitation frequency analysis were compared to NOAA’s 1981–2010 climate normals. The comparison shows consistency with climate normals for the drier part of the state and significant deviations from normals for the wet areas and some major cities. For example, Houston and Beaumont recorded 104 and 114 rainy days from NOAA climate normals but according to the Stage IV data, the two cities witnessed 144 and 128 rainy days on average, respectively, during the study period. Seasonal analysis reveals that the entire state witnesses a higher precipitation frequency in the summer, especially the northern part of the Gulf Coast. In the winter, the entire area that borders the state of Louisiana (Piney Woods region) has a markedly higher precipitation frequency than the rest of the state. April was found to be the driest month across the state, with only 23 wet hours, and September the wettest, with 48% more wet hours than April. The diurnal cycle of the precipitation frequency indicates two peaks of frequency: the wettest hours of the day to be from 3:00 PM to 5:00 PM Central Standard Time (CST) and another, smaller, early morning peak. Trend analysis shows precipitation is becoming more frequent over parts of Texas (about 16.1% of the state), in which the majority of the trend is seen in the dry western region. Detailed analysis of the radar precipitation records can provide useful information about storm characteristics and recent change and variability of precipitation at high spatial resolutions. The data generated in this study, given their spatiotemporal resolutions, can be used to reconstruct the long-term regional climatology of precipitation and hydrology using well-known statistical extrapolation and merging techniques. Accurate estimation precipitation frequency can enable the prediction of wet weather impacts, including interruption of numerous activities such as construction, traffic congestion, and delays, and elevated accident rates [e.g., [33]]. Moreover, how often the ground surface stays wet provides crucial information for the design of roads and hydraulic structures and can be used to determine the expected average antecedent moisture needed to estimate hydrologic parameters such as the well-known curve numbers [e.g., [35]]. The main limitation of the study is the relatively short data record used. The discontinuity in the data due to the operational processing at the radar site may have slightly affected the results, especially in the western region. There is a significant gap in coverage of the NEXRAD radar network in the western part of Texas. The frequencies described are contingent upon the radar precipitation threshold used and the NEXRAD products’ accuracy. Due to the inherent nature of precipitation, the results are expected to be quite sensitive to both the spatial scales of the data and its temporal resolution; however, we believe that the resolutions used provide realistic values.

Author Contributions: H.O.S. developed the research methodology (with input from D.G.), guided this research, and contributed significantly to the preparation of the manuscript for publication. D.G. downloaded and processed the remote sensing products. D.G. developed the scripts used in the analysis and performed the statistical analysis. D.G. prepared the first draft. H.O.S. performed the final overall proofreading of the manuscript. All authors have read and agreed to the published version of the manuscript. Water 2020, 12, 1378 15 of 16

Funding: This research was partially funded by the Texas General Land Office (Project No. 19-250-000-b720). Acknowledgments: The University of Texas at San Antonio is gratefully acknowledged for partially supporting D.G. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analysis, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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