reservoir system management, water allocation, and supply reliability

Journal of Hydrology 300 (2005) 100–113 www.elsevier.com/locate/jhydrol

Modeling river/reservoir system management, water allocation, and supply reliability Ralph A. Wurbs Department of Civil Engineering, Texas A&M University, College Station, TX 77843, USA Received 29 August 2002; revised 3 June 2004; accepted 3 June 2004

Abstract The state of Texas has implemented a modeling system for assessing the availability and reliability of water resources that consists of a generalized simulation model called the Water Rights Analysis Package (WRAP) and input datasets for the state’s 23 river basins. Reservoir/river system management and water allocation practices are simulated using historical naturalized monthly streamflow sequences to represent basin hydrology. Institutional systems for allocating streamflow and reservoir storage resources among numerous water users are considered in detail in evaluating basinwide impacts of water management decisions. The generalized WRAP model is a flexible tool that may be applied to river basins anywhere. The Texas experience in implementing a statewide modeling system illustrates issues that are relevant to water management in many other regions of the world. q 2004 Elsevier B.V. All rights reserved. Keywords: River basin management; Water rights; Water supply reliability

1. Introduction Modeling and analysis methods for evaluating the water supply capabilities of reservoir/river systems are fundamental to the effective management of the highly variable water resources of a river basin. Both hydrologic and institutional considerations are important in assessing water availability and reliability. Analysis methods must deal with the stochastic nature of streamflows and other pertinent variables. River basin management and associated water availability modeling (WAM) involve complex interactions between multiple types of use E-mail address: [email protected] 0022-1694/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2004.06.003

and numerous water users within a framework of various water allocation arrangements and configurations of storage and conveyance facilities. The model development effort reported by this paper integrates institutional and hydrologic considerations reflecting basinwide interconnections in assessments of water availability required to support practical water management decisions. The objectives of the paper are: (1) to outline the general modeling strategy and methods incorporated in the Water Rights Analysis Package (WRAP) and (2) to share experience gained as Texas implemented the modeling system. The river basins of Texas represent a broad diversity of geography, climate, hydrology, and water management practices. Providing flexible

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modeling capabilities for a broad range of practical applications was a governing concern in developing the generalized WRAP modeling system. McMahon and Mein (1986), Votruba and Broza (1989), Wurbs (1993, 1996), ReVelle (1999) and Nagy et al. (2002) provide general reviews of modeling techniques for analyzing reservoir/river system yield and reliability. The US Bureau of Reclamation maintains a Hydrologic Modeling Inventory at http://www.usbr.gov/hmi/ that provides summary descriptions of a number of generalized reservoir/river system simulation models including MODSIM developed at Colorado State University, MIKE BASIN developed by the Danish Hydraulic Institute, RiverWare developed jointly by the Bureau of Reclamation, Tennessee Valley Authority, and University of Colorado, and WRAP described by this paper. A comparative evaluation of models was performed by the Texas Natural Resource Conservation Commission (TNRCC), its partner agencies, and contractors in the process of selecting a generalized model for implementation in Texas (TNRCC, 1998). The WRAP model was adopted for the following reasons. WRAP algorithms are organized based upon a generalized system of assigning priorities that allows flexibility in simulating a prior appropriation water rights permit system and other water allocation schemes. The public domain software can be freely shared by all interested agencies and firms. The generalized model could be readily expanded and improved as the 23 individual river basins of the state were modeled. Although initial versions of WRAP date back to the 1980s, the model has been greatly expanded since 1997 as it has been applied in Texas. Model improvements have focused on two primary areas of complexity: (1) compiling and managing voluminous data and (2) modeling a diverse array of water management practices. WRAP simulates management of the water resources of a river basin or multiple-basin region under a priority-based water allocation system. The model facilitates assessment of hydrologic and institutional water availability for existing and proposed new requirements for instream flows, water supply diversions, hydroelectric energy generation, and reservoir storage. In WRAP terminology, these water use requirements

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and management practices are called water rights. Basinwide impacts of water resources development projects and management strategies are evaluated. The model is generalized for application to any river/reservoir/use system, with input files being developed for the particular river basin of concern. The TNRCC was renamed the Texas Commission on Environmental Quality (TCEQ) in 2002. During 1997 – 2004, the TNRCC/TCEQ, in collaboration with other agencies, engineering firms, and university researchers, implemented a statewide WAM system consisting of the generalized WRAP model and input datasets for each of the river basins of the state. WAM information for Texas including the WRAP datasets is available through the TCEQ’s web site (http://www.tceq.state.tx.us/). The Texas Water Resources Institute (http://twri.tamu.edu) distributes WRAP related research reports. The latest versions of the software and reference and users manuals (Wurbs, 2003a,b) may be downloaded from http://ceprofs. tamu.edu/rwurbs/wrap.htm. For river basins in Texas, the basic input datasets have been developed, and model application involves modifying the input data to reflect alternative water management and use scenarios of concern. Application of the generalized WRAP model outside of Texas requires developing input datasets for the river basins of concern. WRAP is being applied in various places. For example, the US Agency for International Development (USAID) is sponsoring use of the WRAP modeling system in Armenia in support of implementation of a new water code recently enacted by the Armenian Government. The USAID sponsored a study tour to Texas for Armenian hydrologists and water managers in 2003, has translated WRAP documentation into Russian, and is assisting in applying the model in Armenia. With growing demands on limited water resources, effective allocation and management of streamflow and reservoir storage have become increasingly important in Texas and throughout the world. Numerous water users share limited water resources. Institutional water availability in Texas is governed by a water rights system with about 8000 currently active permits that reflect a historical evolution of water allocation practices over several centuries; five different interstate river basin compacts; treaties between the United States and Mexico with

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subsequent implementing agreements; federal, state, and local ownership of reservoir projects; and numerous contracts and agreements between an array of water suppliers and users. Natural hydrology is characterized by great spatial and temporal variability, including the extremes of droughts and floods. Reservoirs are essential to develop dependable water supplies. Reservoir/river system operating policies and practices are often complex.

2. Modeling water availability in Texas Texas encompasses 685 000 km2 and has a population of 21 million people. Climate, geography, and water management vary dramatically across the state from the arid west to humid east, from sparsely populated rural regions to the Dallas-Fort Worth, Houston, and San Antonio metropolitan areas. Mean annual precipitation varies from 20 cm at El Paso on the Rio Grande to 140 cm in the lower Sabine River Basin. Major rivers are shown in the map of Fig. 1. Population and economic growth combined with depleting groundwater reserves are resulting in ever increasing demands on surface water resources

throughout the state (Texas Water Development Board, 2002). The Texas water availability modeling system consists of the generalized WRAP model, hydrology and water rights input data sets for all of the river basins of the state, a geographic information system (GIS), and other supporting data management systems. Texas has 15 major river basins shown in Fig. 2 and eight smaller coastal basins along the Gulf of Mexico between the lower reaches of the major river basins. The WAM system includes WRAP datasets covering the entire state subdivided by river basins, but a few datasets include two river basins. The Guadalupe and San Antonio River Basins are combined in a single model. The Brazos – Colorado Coastal Basin is combined with the Colorado River Basin. The Brazos River Basin model includes the Brazos – San Jacinto Coastal Basin. Information describing the models is provided in Table 1. The river basins of Texas include some basins contained entirely within the state and others shared with neighboring states. The Rio Grande is shared with Mexico. The Texas WAM system is designed for assessing water availability in Texas. However, for the interstate and international river basins, hydrology and water management in neighboring

Fig. 1. International, interstate, and intrastate rivers.

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Fig. 2. River basins of Texas.

states and Mexico are considered in assessing water availability in Texas. The basin models contain the 3365 reservoirs for which a water right permit has been issued. Permits are required to store more than 247 000 m3. Over 90% of the total capacity of the 3365 reservoirs is contained in the 210 reservoirs that have conservation capacities exceeding 6 170 000 m3. The three largest reservoirs are Toledo Bend on the Sabine River, International Amistad on the Rio Grande, and Sam Rayburn on the Neches River with conservation storage capacities of 5.5, 4.3 and 3.6 billion m3. Modeling is complicated by numerous reservoirs, but most of the storage is contained in a few very large reservoirs. Modeling complex operating rules for the larger multiple-reservoir systems is a key concern. The Texas Legislature enacted comprehensive water management legislation in 1997 that authorized development of the WAM system and established a process of regional water resources planning.

The TCEQ is the lead agency for developing and maintaining the WAM system in conjunction with administration of the state’s water rights permit system. The Texas Water Development Board (TWDB) is the lead agency for regional and statewide planning activities, which also involve application of the WAM system. An important lesson learned from the Texas experience is that water rights regulatory programs and water resources planning activities are integrally connected. The WAM system supports a broad range of water management activities and helps to integrate those activities. The TCEQ in collaboration with the TWDB and water management community developed the WAM system during 1997 – 2004 pursuant to the 1997 legislative directive. Consulting engineering firms and university researchers under contract with the TCEQ performed most of the technical work. Consulting firms developed WRAP input data sets and modeled specified water management scenarios

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Table 1 River basin models in the TCEQ Texas WAM system Major river basin or coastal basin

Area in Texas (km2)

Brazos River Canadian River Colorado River Cypress Bayou Guadalupe-San Ant Lavaca River Neches River Nueces River Red River Rio Grande Sabine River San Jacinto River Sulphur River Trinity River

115 000 32 900 108 000 7280 26 500 5980 25 900 43 900 63 400 125 000 19 200 14 500 9220 46 500

Coastal Basins Colorado-Lavaca Lavaca-Guadalupe Neches-Trinity Nueces-Rio Grande San Antonio-Nueces Trinity-San Antonio

2440 2590 1990 27 000 6860 648

Area outside Texas (km2)

6660 90 00 5100 259 – – – – 61 000 347 000 6040 – 492 – – – – – – –

Number of

Storage capacity ( £ 106 m3)

Control points

Water rights

Reservoirs

3818 85 2263 158 1334 176 304 544 443 974 373 386 77 1329

1606 56 1591 132 853 71 327 376 447 2562 308 164 82 1176

650 47 503 85 233 22 175 122 240 90 206 111 51 702

105 68 216 197 49 83

26 10 134 105 12 21

10 – 31 64 9 14

for each of the river basins. Six initial river basins were modeled in 1997 –1999 as mandated by the Legislature, 16 basins were modeled during 1999 –2002, and the final basin, the Rio Grande, was modeled in 2002 –2004. The WRAP software and data files are publicly available for further modeling studies supporting various water management activities. The many regulatory responsibilities of the TCEQ include administering a water rights permit system. River authorities, irrigation districts, municipal water districts, cities, private companies, and individual citizens hold about 8000 permits to use the surface waters of the state. Changes in water use or management practices or development of new water projects require TCEQ approval of either new permits or revisions to existing permits. In evaluating permit applications, the TCEQ must (1) determine whether sufficient water is available to supply the proposed new use and (2) evaluate the impacts on all other

Analysis period

Mean flows

Unappropriated

Naturalized ( £ 106 m3/yr) 5758 1192 5878 1078 997 290 4818 1284 4965 16 149 7873 787 930 9254

1940–1997 1948–1998 1940–1998 1948–1998 1934–1989 1940–1996 1940–1996 1934–1996 1948–1998 1940–1900 1940–1998 1940–1996 1940–1996 1940–1996

7845 235 3701 2154 2593 1203 7694 1071 19,173 5350 8499 2723 3083 8489

5583 220 1300 1598 2522 978 5589 14739 18,623 1220 4320 2279 2562 5258

67

1940–1996 1940–1996 1940–1996 1948–1998 1948–1998 1940–1996

167 194 749 307 697 223

150 192 649 302 695 207

– 40 140 2 6

water users in the river basin. In conjunction with developing the WAM system, the TCEQ has informed all water right permit holders of the reliabilities associated with their existing permits. TCEQ procedures require that water management entities and their consultants use WRAP in preparing water right permit applications. TCEQ staff uses the model in evaluating permit applications. The generalized WRAP model and Texas WAM System are motivated by the water rights permit system but support a broad range of water resources planning and management activities. The TWDB, regional planning committees, and consulting firms are systematically applying the modeling system in regional and statewide planning studies mandated by the Legislature. River authorities and other entities use the model to develop water management plans and to evaluate particular projects. Agencies, consulting firms, and university researchers continue to find new types of modeling applications.

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3. Water rights analysis package (WRAP) model The WRAP model was developed at Texas A&M University, with early versions dating back to the mid-1980s. The model has been greatly expanded and improved since 1997 in conjunction with the TCEQ-administered statewide modeling effort. Model development has been an evolutionary process with extensive interactions between professionals from the agencies and consulting firms applying the model to specific river basins and university researchers responsible for improving the modeling methodology and computer software. WRAP is a set of Fortran programs called WinWRAP, HYD, SIM, and TABLES. WinWRAP is a user interface for executing the programs within Microsoft Windows. HYD is a set of computational routines for converting gauged streamflows to naturalized flows and compiling sets of net reservoir evaporation less precipitation depths. HYD output consists of hydrology input files for SIM. Program SIM performs the river/reservoir system water allocation simulation. TABLES organizes the simulation results and develops frequency relationships, reliability indices, and summary statistics.

4. Overview of modeling methodology WRAP simulates capabilities for meeting specified water management and use requirements during a hypothetical repetition of historical hydrology. Water managers are concerned with future not past hydrologic conditions. However, since the future is unknown, historical hydrology is used to capture the hydrologic characteristics of a river basin. The water management/use scenario might be actual current water use, projected future conditions, the premise that all current permit holders use their full-authorized amounts, or some other scenario of interest. The TCEQ WAM system includes simulations for various combinations of water use, return flows, and conditions of reservoir sedimentation. The results shown in this paper are for simulations based on the premises that all permit holders use the full amounts authorized by their permits, minimal return flows, and present conditions of reservoir sedimentation. This

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is the simulation scenario used by the TCEQ in evaluating water right permit applications. A particular application might focus on evaluating reliabilities of existing or proposed reservoirs and conveyance facilities to supply water uses authorized by current water right permits, with basin hydrology represented by sequences of monthly naturalized streamflows and reservoir net evaporation less precipitation depths at all pertinent locations for each of the 720 months of a 1940– 1999 hydrologic period-of-analysis. The model allocates water to meet the current water use requirements during each sequential month of the 720-month simulation. Reliability indices and frequency relationships are developed from the simulation results. Simulation results include monthly sequences of naturalized, regulated, and unappropriated flows, reservoir storage contents, reservoir net evaporation volumes, water supply diversions and shortages, hydroelectric energy generated and shortages, and other variables. Diversion and hydropower shortages are target amounts less actual amounts as limited by water availability. Naturalized streamflows are flows that would have occurred without the water users and facilities reflected in the WRAP water rights input dataset. Regulated flows are physical flows at a location after considering all water rights. Unappropriated flows represent water still available for further appropriation. Unappropriated flows may be less than regulated flows due to some of the water being committed to instream flow requirements at that location or committed to other diversion, storage, and instream flow rights at downstream locations. The spatial configuration of a river/reservoir/use system is modeled as a set of control points. All system components are assigned control point locations. Essentially any configuration of stream tributaries and conveyance systems may be modeled. Naturalized, regulated, and unappropriated flows are developed for all control points. Algorithms in the model are based on each control point having its next downstream control point defined in the input. The number of control points adopted for each of the Texas basin models are listed in Table 1 and range from 49 to 3818. The complexity of a model can vary greatly depending on its purpose. During the early development of WRAP, Wurbs et al. (1994) modeled

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Fig. 3. Outline of WRAP simulation.

the Brazos River Basin with 19 control points. The purpose of the study was to analyze a system of 12 reservoirs operated by the Brazos River Authority, with conservative approximations of the impacts of the numerous other water users in the basin on this system. The Brazos River Basin dataset available at the TCEQ WAM web site has 3818 control points. Based on this model, each of the over 1200 permit holders in the basin was provided information regarding reliabilities associated with each of their individual water rights. Both the 19 and 3818 control point models of the Brazos River Basin fulfilled their intended purposes but represent dramatically different levels of modeling detail. River basins outside of Texas have been modeled with just a few control points. A WRAP simulation combines information describing natural hydrology and human water management. Hydrology is represented by monthly naturalized streamflows spanning periods of many years that reflect the hydrologic characteristics of the river basin including severe droughts. Hydrology also includes net evaporation less precipitation rates from reservoir water surfaces. In WRAP, a water right is a set of water management and use requirements. A typical water right could include water supply diversion or hydroelectric energy generation

requirements and storage in any number of reservoirs. In the Texas WAM system, model water rights correspond directly to water right permits, but many of the more complex permits are modeled with multiple model water rights. Thus, the 10 059 model water rights noted in Table 1 is greater than the approximately 8000 actual water right permits. Environmental instream flow requirements are modeled as a special type of water right. Water use targets vary seasonally over the 12 months of the year and may also vary as a function of storage and streamflow conditions. As outlined in Fig. 3, the modeling process includes the following steps. 1. Sequences of naturalized monthly flows covering the hydrologic period-of-analysis are developed by adjusting gauged flows to remove the effects of human water management. 2. A specified scenario of water management and use requirements is simulated for each sequential month of the hydrologic period-of-analyses using the naturalized streamflow sequences as input. 3. The voluminous simulation results are organized and summarized by developing frequency relationships, reliability indices, and other summary statistics.

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Step 1 begins with sequences of measured flows at stream gauging stations. These gauged flows are adjusted to remove the historical effects of constructing and operating water control facilities and past water use. Gaps in gauge records are reconstituted by regression with naturalized flows at other gauges. The WRAP input data sets available through the Texas WAM System include sets of naturalized flows at about 500 gauging stations. However, the models listed in Table 1 include 12 982 control points. The naturalized streamflows at the gauges are transposed to ungauged sites of interest, optionally either in step 1 or 2. Naturalized flows synthesized for ungauged sites in step 1 are included in the input data for the step 2 simulation. Within the system simulation of step 2 of Fig. 3, computations step through time. Within each sequential month, the computations are performed in a water rights loop, with each set of water use requirements (water right) considered in priority order. An available streamflow array is maintained for all locations (control points). As each water right is addressed in priority order, the following tasks are performed. (a) The amount of water available to the water right is determined based on the available streamflow array at its location and all downstream locations. (b) Water use requirements are satisfied subject to water availability, and water accounting computations are performed. Reservoir evaporation and hydropower computations necessitate an iterative algorithm. (c) The available flow array is adjusted for that location and all downstream locations to reflect the effects of the water right. Selected portions of the simulation results are stored in one or more output files. A post-processor program reads the output file, organizes the simulation results, and develops reliability indices, frequency relationships, and summary displays in user-specified formats. The conventional modeling approach outlined above and adopted for the TCEQ WAM system is designed for long-term planning studies and preparation and evaluation of water right permit

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applications. A new conditional reliability modeling (CRM) component expands WRAP capabilities to include evaluation of reliabilities of meeting water needs during the next relatively short period of time, typically ranging from a month to a year (Salazar, 2002). These short-term reliabilities are highly dependent on initial storage contents. For specified initial storage conditions, CRM develops estimates of reliabilities of meeting water use requirements during a user-specified time period and exceedence frequency versus end-of-period storage relationships. The added CRM features provide a decision-support tool for formulating reservoir system operating policies, developing operating plans for the next season or year, administering water rights during droughts, and related applications that are highly dependent on preceding storage. In the WRAP – CRM, the long sequences of monthly naturalized flows and net evaporation rates are divided into many short sequences. For example, a 1940– 2000 sequence may be divided into 61 annual sequences. The system is simulated 61 times with 61 different flow sequences, with each simulation starting with the same storage content. Reliability measures are developed from the simulation results. The WRAP – CRM incorporates a method for assigning exceedence probabilities to each streamflow sequence as a function of preceding storage.

5. Natural hydrology Hydrology in WRAP consists of sequences of naturalized monthly streamflows at all control points and monthly net evaporation less precipitation depths for all reservoirs. The 1940 – 1997 monthly naturalized flows at US Geological Survey gauge 08111500 on the lower Brazos River about 180 km above its mouth on the Gulf of Mexico are plotted in Fig. 4 to illustrate the great random variability that characterize rivers in Texas. The means of the naturalized flows at the outlet of each basin are tabulated in Table 1. The hydrologic period-of-analysis must be sufficiently long, reflecting a full range of fluctuating wet and dry periods, to allow simulation results to be used to develop frequency relationships, reliability indices, and other statistics that characterize the water

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Fig. 4. Monthly naturalized flows of the Lower Brazos River.

supply capabilities of a river basin. The hydrologic simulation periods adopted for each of the TCEQ WAM models are listed in Table 1. For most of the state, the most hydrologically severe drought of record occurred during 1951– 1957, ending in April 1957 with one of the greatest floods on record. The most economically damaging drought was in 1995 –1996. 5.1. Gauged flows converted to naturalized flows The objective of streamflow naturalization procedures is to develop a homogeneous set of flows representing a specified condition of river basin development The extent to which observed historical flows are adjusted varies depending on circumstances. For relatively undeveloped watersheds, little or no adjustments may be necessary. In extensively developed river basins, adjusting for the effects of all human activities is not feasible. Naturalized flows are typically developed by adjusting recorded flows at gauging stations to remove the impacts of major upstream reservoirs, diversions, return flows from surface and ground water sources, and possibly other factors. At a given gaging station, for a particular month during the historical record, the naturalized monthly flow volume QN is computed as X X X X QN ¼ QG þ D 2 RF þ E þ DS ð1Þ where QG is the gauged flow at the site; D; the water supply diversions from the river system upstream of the gauge; RF, the return flows into the river system upstream of the gauge; E; the net evaporation from reservoirs located upstream of the gauge; DS is change

in storage in upstream reservoirs. Net evaporation E is the volume of evaporation from a reservoir water surface minus the proportion of the precipitation volume falling on the water surface that would not have reached the stream in the absence of the reservoir. As indicated by the summation signs in Eq. (2), many reservoirs, diversions, and return flows may be located upstream of the gaging station. The monthly adjustments vary historically over the period-ofanalysis as new reservoir projects and other water control facilities were constructed and water use practices changed. The larger river basins in Texas contain numerous smaller reservoirs, but most of the storage capacity is contained in a relatively few large reservoirs. Judgments are required regarding which of the smaller reservoirs to include in the adjustments. Other types of adjustments may also be made. For example, in modeling the San Antonio and Guadalupe River Basins, changes in spring flows associated with aquifer management plans, simulated with a groundwater model, are reflected in WRAP as adjustments to naturalized streamflows. Gauge records often do not cover the entire periodof-analysis. A particular gauge may have been installed later than the others or terminated or gaps may exist in the recorded data. Naturalized flows for months with missing records are reconstituted typically by multiple linear regression analysis with naturalized flows at other gaging stations in the same vicinity. 5.2. Naturalized flows transposed from gauged to ungauged sites WRAP includes several alternative methods for transferring naturalized flows from gauged to ungauged sites (Wurbs and Sisson, 1999) Most applications in Texas have used an option based on the Natural Resource Conservation Service (NRCS) relationship between precipitation depth P and runoff depth Q (NRCS, 1985) 8 2 > < ðP 2 0:2SÞ VR ¼ P þ 0:8S > : 0

if P $ 0:2S if P . 0:2S

ð2Þ

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25 400 2 25:4 ðVR ; P; S in cmÞ CN 1000 2 10 ðVR ; P; S in inchesÞ S¼ CN

S¼

where VR is the runoff volume-equivalent resulting from a precipitation depth P: VR and P in cm or inches are multiplied by the drainage area and a conversion factor to obtain volumes. The maximum potential watershed retention S is expressed in terms of a curve number CN, which is a dimensionless parameter ranging from 0 to 100. The CN is a watershed parameter reflecting land cover and soil type. A CN of 100 represents a limiting condition of a perfectly impervious watershed with zero retention and thus all of the rainfall becoming runoff. Smaller values for the CN represent watersheds that abstract more of the rainfall. The NRCS developed the CN method for predicting the runoff volume to result from rainfall events. However, WRAP applies the CN method in a manner that is quite different from conventional usage. Given the naturalized monthly flow at the gauge, precipitation P is computed by the NRCS equation with the CN for the gauged watershed. After adjusting the P by a long-term mean annual precipitation ratio, it is substituted back into the NRCS equation with the CN for the ungauged watershed to determine the flow at the ungauged site. If the CN and long-term mean precipitation are the same for the gauged and ungauged watersheds, this method reduces to simply distributing streamflow in proportion to drainage area. The TCEQ WAM system includes an ArcGIS-based system for determining the drainage area, CN, and mean annual precipitation for all pertinent watersheds (Maidment, 2002; Goplan 2003).

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volumes. The TCEQ WAM System uses a database maintained by the TWDB of precipitation rates and reservoir evaporation rates for each month from 1940 to the present for each of 75 one-degree quadrangles covering the state. An alternative approach is to adopt mean net evaporation depths for each of the 12 months of the year for each reservoir. 5.4. Channel losses Channel losses may be considered in modeling the effects of diversions, return flows, and reservoirs on streamflows at downstream locations The flow change at the downstream end of a stream reach DQDS is related to the change at the upstream end of the reach DQUS as follows DQDS ¼ ð1:0 2 FCL ÞDQUS

ð3Þ

where FCL is a dimensionless channel loss factor, which is provided as model input. Eq. (1) may be repeated any number of times to translate an adjustment through multiple reaches. For many of the stream reaches in the Texas WAM System, channel losses are considered negligible and not incorporated in the model. Channel losses are significant and included in the data sets for many stream reaches. Channel loss factors have been developed in terms of loss per unit length based on studies of water balances for reaches between gauging stations. Runoff entering the stream between the gaging stations is estimated using rainfall records combined with the Natural Resource Conservation Service (1985) CN based rainfall – runoff relationship.

6. Water management and use 6.1. Water allocation systems

5.3. Net reservoir evaporation depths WRAP hydrology input also includes net evaporation less precipitation depths from reservoir water surfaces The model adjusts the depths for the precipitation runoff from the reservoir site that is already reflected in the naturalized streamflows. The net evaporation depths are used by the model in combination with reservoir storage versus surface area relationships to compute net evaporation

In Texas, like elsewhere in the world, streamflow and reservoir storage is allocated between nations, states, water supply entities, and numerous water users. Water rights in Texas evolved historically over several centuries into an unmanageable mixed system of several versions of riparian and appropriative rights. The Water Rights Adjudication Act of 1967 initiated a 20-year adjudication process to merge all water rights in Texas, except the Rio Grande, into

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a uniform prior appropriation permit system. A somewhat different permit system was established for the Texas portion of the Rio Grande. The TCEQ administers the water right permit systems. Water availability is also governed by contractual agreements between cities, water districts, river authorities, industries, and federal reservoir management agencies. Texas participates in five interstate river compacts administered by commissions representing the member states. The flow of the Rio Grande and water stored in Falcon and Amistad Reservoirs are allocated between the United States and Mexico in accordance with 1908 and 1944 treaties. The generalized WRAP has flexible capabilities for modeling these water allocation systems. The WRAP simulation algorithms are based on allocating available streamflow to each water right in turn in ranked priority order. Diversion, instream flow, hydropower, and storage refilling targets for each right are met to the extent allowed by available streamflow and storage prior to considering the requirements of more junior rights. In the Texas prior appropriation water rights system, priority numbers typically represent dates specified in the permits. These dates are based on historical water use for old rights and the dates the permits were issued for more recent rights. Senior water users are protected from having their supplies diminished by later appropriators. The priority-based simulation approach in WRAP is essential for the Texas WAM System and provides flexibility that may be readily applied elsewhere as well. Model-users, with a little ingenuity, can devise various schemes for assigning priority numbers to model relative priorities for allocating water. A priority scaling option in the model allows rights associated with specified water use types to be conveniently adjusted. For example, all municipal rights could be given priority over all agricultural rights in a particular simulation run. Another option allows water demands to be met in upstream to downstream order without regard to priorities. 6.2. Reservoir/river system operations Water management and use requirements, policies, practices, and facilities are described in terms of water

rights. The model provides flexibility for modeling complex system configurations and operations. Extensive improvements to WRAP have been made in response to various situations encountered as the individual river basins were modeled. The objective was to develop a generalized model providing the flexibility needed to address the diverse water management practices found across the state. Required and optional features for defining a water right include: † identifiers for aggregating simulation results for groups of related rights † locations of system components by control point † priority specifications † diversion, instream flow, and hydroelectric energy targets for the 12 months of year † specifications for varying water use targets as a function of storage or streamflow † seasonal or annual limits on diversions, reservoir releases, or streamflow depletions † return flow specifications in various optional formats † conveyance of flow through pipelines and canals † active and inactive reservoir storage capacity † reservoir storage volume versus surface area and elevation relationships † river/reservoir system operating rules including multiple-reservoir system operations, multipleowner reservoirs, off-channel storage, and constraints on depleting streamflows Multiple-reservoir system operating rules are based on balancing storage depletions in specified zones of each reservoir. WRAP provides various options for modeling operating policies involving multiple users sharing the same reservoir or multiplereservoir system. Reservoir water surface elevation versus storage volume and surface area relationships are readily available for larger reservoirs, but are difficult to obtain for numerous smaller reservoirs. Data on loss of storage capacity by sedimentation is limited, even for larger reservoirs. In developing the data sets for the Texas WAM System, elevation – storage – area tables were obtained for each individual reservoir for over 200 large reservoirs, which account for most of the total storage capacity. Generalized storage – area

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relationships were adopted for over 3000 smaller reservoirs. Storage capacities for all of the reservoirs are cited in their water right permits.

Table 2 Reservoir storage–frequency relationships by River Basin River Basin

Exceedance frequency Min

7. Measures of water availability and reliability WRAP simulation studies are organized in various ways to develop an understanding of river basin systems of concern and to support decision-making processes. Alternative simulations demonstrate the effects of alternative water use scenarios, operating plans, and proposed construction projects. Simulation results may be organized in various formats including the entire time series of monthly or annual values of selected variables, water budgets, frequency statistics, and reliability indices. The results of a simulation are typically viewed from the perspectives of frequency, probability, percent-of-time, or reliability of meeting water supply, instream flow, hydropower, and/or reservoir storage targets. 7.1. Frequency relationships and reliability indices Exceedence frequency relationships are developed for naturalized, regulated, and unappropriated flows at locations of concern and for storage or storage draw-downs for reservoirs Exceedance frequency ¼

n ð100%Þ N

ð4Þ

where n is the number of months during the simulation that a particular flow or storage amount is equaled or exceeded and N is the total number of months in the simulation. Summary statistics also include monthly and annual means, standard deviations, minima, and maxima. Table 2 provides a tabulation of the total basin storage levels that are equaled or exceeded at several specified frequencies. The storage – frequency relationships in Table 2 are for the total storage in all reservoirs included in the models for each river basin. The total number of reservoirs and total conservation storage capacity are listed in Table 1. For example, the conservation storage capacities of the 650 reservoirs in the Brazos River Basin model total 5758 million m3. The total end-of-month storage content equals or exceeds 4765 million m3 for 25% of

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Brazos Canadian Colorado Cypress Guadalupe-SA Lavaca Neches Nueces Red Sabine San Jacinto Sulphur Trinity

90%

75%

50%

25%

10%

Total reservoir storage content ( £ 106 m3) 1849 3156 3886 4390 4765 5110 8 10 35 375 723 1062 772 1805 2883 3483 4163 4522 323 705 810 904 965 1001 14 398 686 882 950 982 14 188 229 264 290 290 1873 3051 3643 4265 4603 4764 4 7 56 452 867 1086 3635 3852 4030 4213 4444 4631 2216 4215 5475 6659 7524 7852 71 406 565 699 767 784 338 601 681 767 824 909 1621 4794 5909 6735 7461 7937

Max

5697 1191 5826 1027 997 290 4808 1267 4953 7873 787 930 9005

the 696 months during the 1940 –1997 hydrologic simulation period. Storage – frequency tables are usually developed for individual reservoirs, rather than basin totals. Total storage in particular multiplereservoir systems may also be of interest. Volume and period reliabilities may be computed for water supply diversions for individual water rights or the aggregation of selected groups of rights. Similar energy reliability indices are computed for hydroelectric energy production targets. Volume reliability is the ratio of the water volume supplied to the demand target or equivalently the ratio of the mean actual rate supplied to mean target rate, expressed as a percentage. Period reliability is the percentage of months in the simulation for which a specified demand target is met. Period reliability is an expression of the percentage of time that the demand can be met or equivalently the likelihood of the demand being met in any randomly selected month. Reliability tables also include tabulations of both the percentage of months and the percentage of years during the simulation for which the amounts supplied equal or exceed specified magnitudes expressed as a percentage of the target. Reliabilities for water supply diversions in the San Jacinto River Basin are shown in Table 3. About 150 public agencies, private companies, and individual citizens hold permits to divert a total of 559 million m3/year of water from the San Jacinto River and its

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R.A. Wurbs / Journal of Hydrology 300 (2005) 100–113

Table 3 Water supply reliabilities for the San Jacinto River Basin

the 57 years, at least 90% of the sum of annual targets is supplied.

Water right permit holder

San Jacinto River authority

City of Houston

All others

7.2. Interpretation of reliabilities

Diversion target ( £ 106 m3/yr) Mean shortage ( £ 106 m3/yr) Volume reliability (%) Period reliability (%)

123.4

207.3

228.5

2.19

0.0

22.7

98.22 98.10

100.0 100.0

90.06 24.56

Percentage of months with diversion equaling or exceeding 100% of target 98.1 100.0 24.6 95% of target 98.1 100.0 69.3 90% of target 98.1 100.0 76.6 75% of target 98.2 100.0 81.3 50% of target 98.2 100.0 98.2 Percentage of years with diversion equaling or exceeding 100% of target 96.5 100.0 0.0 95% of target 96.5 100.0 49.1 90% of target 96.5 100.0 64.9 75% of target 98.2 100.0 91.2 50% of target 98.2 100.0 98.2

tributaries. The City of Houston and the San Jacinto River Authority (SJRA) are the largest permit holders with 37 and 22%, respectively, of the total diversion amount. Houston is also the largest purchaser of water from the SJRA under permits held by the SJRA. Lake Houston owned by the city and Lake Conroe owned by the river authority account for 23 and 67% of the 787 billion m3 of water supply storage capacity in the 111 reservoirs located in the river basin. For the simulation results presented in Table 3, the water supply diversion rights are divided into three groups: (1) the several rights held by Houston, (2) those held by the SJRA, and (3) the approximately 150 other smaller rights. Table 3 shows that the City of Houston rights, with senior priorities and ample storage capacity, are fully met without shortage during the 1940 – 1996 simulation. The mean of the monthly SJRA diversion amounts is 98.22% of the mean authorized (target or permitted) amount. The last column of Table 3 presents reliabilities for the aggregation of all the other rights. The total target amount for all of these other rights is fully satisfied during 24.6% of the 684 months of the simulation. During 76.6% of the months, at least 90% of the sum of monthly targets is met. During 64.9% of

The reliabilities computed by WRAP provide meaningful information but are subject to interpretation. The shortages represent a general index of supply failures that could involve emergency demand management measures, negotiation of resource reallocations, or similar actions. Although the Texas water rights system and the WRAP model are based on protecting senior water users, in actual situations involving insufficient water supply, users share the shortages to some degree regardless of the relative seniority of their rights. Temporary demand management measures are implemented. Water allocations during drought depend on political negotiations, alternative demand management and supply augmentation measures available to different entities, and other factors in addition to the water rights permit system. Development of plans for actions to be taken during periods of water shortage is receiving increased attention. In evaluating permit applications, the TCEQ has applied a general rule that municipal supplies should have a volume and period reliability of 100%, and for agricultural supplies, 75% of the permitted demand should be met at least 75% of the time. These guidelines are subject to exceptions and future refinement. Criteria for defining unacceptable levels of impact of a proposed plan on the reliabilities of other water users throughout a river basin are also evolving as experience is acquired in applying the modeling system. The Texas WAM studies indicate that reliabilities are not very sensitive to changes in demand targets. Conversely, the amounts that may be supplied change greatly with relatively small changes in reliability requirements. The amount of water supplied from Texas river systems can be increased significantly by accepting higher risks of shortages or emergency demand reductions. Reliabilities are also highly dependent on reservoir storage capacity and multiple-reservoir/river system operating rules. The Rio Grande and major reaches of other rivers in the dry western half of the state are overappropriated. Streamflow in several major urban

R.A. Wurbs / Journal of Hydrology 300 (2005) 100–113

regions with wetter climates is also either completely appropriated or nearly so. The TCEQ will not issue permits for additional water use from these river reaches. Marketing or transferring of existing water rights among users is encouraged. For other rivers, water is still available for further appropriation. The TCEQ issues or modifies numerous water right permits each year.

8. Summary and conclusions The generalized WRAP modeling system is designed for assessing capabilities for river/reservoir systems to satisfy municipal, industrial, and agricultural water supply, hydroelectric power, environmental instream flow, and reservoir storage needs Water availability and reliability for a proposed new or modified water use, management strategy, or water development project, and the impacts on other water users in the river basin are evaluated. Needs for flexible capabilities for modeling a comprehensive spectrum of water allocation and management practices drove the evolution of WRAP. The state of Texas implemented a WAM system during 1997 – 2004 pursuant to milestone water management legislation enacted by the Legislature in 1997. The generalized modeling capabilities reflected in WRAP have been expanded as necessary to address the broad array of water management practices and issues found in the diverse river basins of the state. Requirements for data describing hydrology, water use, and water control facilities are a governing concern. The river basin models for Texas represent a major effort to develop a modeling system accessible by a diverse water management community for shared use in a broad range of activities. Planning and regulatory agencies, river authorities and other water supply agencies, and consulting engineering firms working for these agencies are applying a common set of tools in an integrated manner. WAM is essential to effective water management. The concept of a generalized modeling system shared by a water management community that reflects the institutional factors governing water management and use is growing in importance. Issues encountered in implementing a modeling system in Texas

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illustrate the complexities involved in river basin management and associated computer modeling throughout the world.

References Goplan, H., 2003. WRAP hydro data model, finding input parameters for the water rights analysis package. Technical Report 233. Texas Water Resources Institute, College Station, TX, 184 pp. Maidment, D.R. (Ed.), 2002. Arc Hydro: GIS for Water Resources. ESRI Press, Redlands, CA, 203 pp. McMahon, G.F., Mein, R.G., 1986. River and Reservoir Yield. Water Resources Publications, Fort Collins, CO, 236 pp. Nagy, I.V., Asante-Duah, K., Zsuffa, I., 2002. Hydrological Dimensioning and Operation of Reservoirs: Practical Design Concepts and Principles. Kluwer, Dordrecht, 225 pp. Natural Resource Conservation Service, 1985. National Engineering Handbook, Section 4: Hydrology. US Department of Agriculture, Washington, DC. ReVelle, C.S., 1999. Optimizing Reservoir Resources. Wiley, New York, NY, 180 pp. Salazar, A.A., 2002. Conditional reliability modeling to support short term river basin management decisions. PhD Dissertation. Department of Civil Engineering, Texas A&M University, College Station, TX, 275 pp. Texas Natural Resource Conservation Commission, 1998. Evaluation of existing water availability models. Technical Paper No. 2, Austin, TX, 63 pp, http://www.tnrcc.state.tx.us. Texas Water Development Board, 2002. Water for Texas-2002, vols. I –III, Document No. GP-7-1, Austin, TX, http:www.twdb. state.tx.us. Votruba, L., Broza, V., 1989. Water Management in Reservoirs. Developments in Water Science. vol. 33. Elsevier, Amsterdam, 444 pp. Wurbs, R.A., 1993. Reservoir system simulation and optimization models. Journal of Water Resources Planning and Management 119(4), 455 –472. Wurbs, R.A., 1996. Modeling and Analysis of Reservoir System Operations. Prentice-Hall, Upper Saddle River, NJ, 356 pp. Wurbs, R.A., 2003a. Water Rights Analysis Package (WRAP) Modeling System Reference Manual. TR-255, Texas Water Resources Institute, College Station, TX, 222 pp. Wurbs, R.A., 2003b. Water Rights Analysis Package (WRAP) Modeling System Users Manual, TR-256. Texas Water Resources Institute, College Station, TX, 139 pp. Wurbs, R.A., Sisson, E.D., 1999. Comparative evaluation of methods for distributing naturalized streamflows from gauged to ungauged sites. Technical Report 179, Texas Water Resources Institute, College Station, TX, 177 pp. Wurbs, R.A., Sanchez-Torres, G., Dunn, D.D., 1994. Reservoir/ river system reliability considering water rights and water quality. Technical Report 165, Texas Water Resources Institute, College Station, TX, 170 pp.