PRUEBA DE ARTES (DANZA Y ARTE DRAMÁTICO)

MODELING PULSE FLOW REQUIREMENTS

The ability of WRAP to simulate individual flow events is improved with the daily time step modeling capabilities in SIMD. Flows occurring as a direct result of a rainfall-runoff event tend to have great fluctuations in flow rates over short time spans. Daily computational time steps improve the accuracy of the simulation with regard to hydrograph representation and water management or flood control response to these events. Modeling capabilities in SIMD addressing flood control operations are discussed in Chapter 5. This chapter introduces modeling capabilities specific to SIMD that address pulse flow events in the context of environmental instream flow needs.

The concept of an environmental instream flow regime is introduced and briefly discussed in this chapter to provide context with regard to the application of the pulse flow modeling capabilities presented later in the chapter. Like all simulation capabilities in WRAP, the specific input records are generalized and may have application in any context outside of the intended use. The PF and PO input records for SIMD are introduced and discussed prior to the pulse flow examples provided at the end of the chapter. The discussion of the PF and PO records includes the mechanics of the computations used during the simulation. The fields, variables, and descriptions for the PF and PO input records are provided in Appendix A. This chapter concludes with two examples of modeling the various types of pulse flow events that might be included in a comprehensive environmental instream flow regime recommendation.

Environmental Instream Flow Regime

IF record instream flow requirements in SIM/SIMD typically involve setting a single

target for regulated low flows. The typical low flow IF record target may vary monthly according to a pattern established with UC records. These IF record applications are generally suitable to model the majority of the historical water right permit requirements that are intended to protect minimal environmental instream flow needs. An example of single low flow requirements for instream flows includes the protection of water quality. Various methods exist for developing single low flow requirements and can include evaluation of stream flow records to determine the minimum annual mean 7-day flow rate with a 2-year recurrence frequency also known as the 7Q2. However, single low flow requirements only address a small portion of the flow variability observed in most stream gage records.

Stream gage records of locations with a meaningful percentage of upstream unregulated drainage area typically exhibit a range of flow rates and rates of change in flow over various periods of time. Poff and Ward (1989) and Richter et al. (1996) characterize the natural flow regime of a stream in terms of magnitude of flow rate, quantity, event duration, event frequency, and other numerical parameters. Throughout this chapter, the term flow regime will be used to refer to the complete host of flow components at a location or within a stream gage record from low to high flows. A flow regime is subdivided into components of the flow regime. Each component has a respective set of numerical parameters that may define flow rates, quantities, durations, and frequencies that are pertinent to the component. The term environmental instream

flow requirement is used to refer to SIMD instream flow IF records that set targets based on the

Four components of a flow regime (TIFP, 2005) are used for developing SIMD environmental instream flow requirements as presented in the simulation examples in this chapter. The four components are subsistence, base, pulse, and overbank flows. Subsistence flows are the lowest magnitude flows used to set instream flow requirements. Subsistence flow requirements are intended to be applied infrequently for the protection of water quality and provide minimally survivable habitat for aquatic species. Base flows might be referred to as normal or average flow conditions that occur between significant rainfall-runoff events. Base flows provide aquatic species with a range of suitable habitats and may also contribute to nutrient and sediment transport. High flow pulse events are a response to rainfall-runoff events. High flow pulses are characterized by short duration events of elevated to high flow rate that remain within the banks of the stream. High flow pulses provide aquatic species with longitudinal connectivity along the stream and help to maintain the physical characteristics of the stream through geomorphic processes. Overbank pulse flows are high flow events that exceed the banks of the stream. Such events provide aquatic species with lateral connectivity between the stream and flood plain and shape the riparian zone as well as the stream channel. Overbank flows are typically considered flooding events and therefore may be constrained to the extent possible by upstream flood control reservoirs and related structures.

Hydrologic analyses of stream flow records to assess environmental flow needs may include consideration for seasonality or hydrologic conditions within the basin. Seasonality may influence the characteristics of base flow or pulse flow components. For example, several months of the year during the spring season may have higher base flow or pulse flow magnitudes than the months considered to be summer or autumn. Hydrologic condition can refer to the state of soil moisture and presence or lack of drought conditions in the drainage area above a stream gage. For the purposes of developing environmental instream flow requirements, the hydrologic condition is used as an indicator used to select between differing levels of flow requirements. Raising or lowering flow requirements based on the hydrologic condition may help to balance the need to preserve flow variability over time while also allowing water availability for human consumption. If the hydrologic condition is indicative of wet conditions upstream of a stream gage, it may be appropriate to set a higher base flow requirement. Conversely, a dry hydrologic condition may indicate a natural tendency for lower base flows. A lower base flow requirement may be similarly protective of environmental needs under a low hydrologic condition.

Within the State of Texas, a series of three omnibus water bills have been signed into law since the late 1990’s relating to water needs for human as well as environmental needs. In 1997, Senate Bill 1 of the 75th Legislature created a comprehensive water supply planning process. In 2001, Senate Bill 2 of the 77th Legislature, among other water related items, created the Texas Instream Flow Program (TIFP) as a jointly administered program of the Texas Commission on Environmental Quality (TCEQ), the Texas Parks and Wildlife Department (TPWD), and the Texas Water Development Board (TWDB). The purpose of the program was "to perform scientific and engineering studies to determine flow conditions necessary for supporting a sound ecological environment in the river basins of Texas" (TIFP, 2005). Most recently in 2007, Senate Bill 3 of the 80th Legislature created a process for developing recommendations to meet instream flow and bay and estuary freshwater inflow needs. The Senate Bill 3 process includes a scientific and stakeholder process which culminates in the adoption of basin specific instream and freshwater inflow standards.

The term environmental flow standard refers to rules adopted by the TCEQ that are supportive of a sound ecological environment while also accounting for public interests. The environmental flow standards are established at various locations within each basin in Texas and are comprised of the components of an environmental flow regime. The environmental instream flow requirements discussed in this chapter refer to SIMD instream flow IF records that set targets based on the defined flow regime components at a particular location. The numerical parameters that define the environmental flow regime components serve as a common connection between the environmental flow standards and the IF record targets that set requirements within the modeling.

SIMD Input Records and Methods for Flow Regime Modeling

Four components of a flow regime are examined in the context of simulation modeling in this chapter. The four components include subsistence, base, pulse, and overbank flows. The qualitative characteristics of each are described in the previous section of this chapter and are further described in detail in Poff and Ward (1989), Richter et al. (1996), Richter et al. (1997), and TIFP (2005). Input records in the DAT file are created for the purpose of setting IF record targets within the simulation that reflect the numerical parameters of the flow regime components. Simulation input records in addition to the IF record are required to address the complexity of the flow regime components at a single stream gage location. Additional input records for modeling components of an environmental flow regime can include, but are not limited to, the following:

• Pulse flow PF and pulse flow option PO records • Flow switch FS or cumulative volume CV records • Target option TO record

• Drought index DI/IS/IP/IM records

• Target setting water right WR records (field 8, option 8)

Pulse Flow PF and Pulse Option PO records

High flow pulses are characterized by short duration events with a high flow rate and may include flooding conditions. These events are a direct response to rainfall-runoff processes. The rising limb of a typical high flow pulse hydrograph climbs sharply to a peak, after which flows recede back to base flow levels. Pulse events are usually considered to be events with duration measured in days or weeks. The unique sub-monthly parameters to be used for initiating, tracking, and terminating high flow pulse events have motivated the creation of a specific set of input records for SIMD. The pulse flow PF record and pulse flow option PO record are discussed in this section. Tables 8.1 and 8.2 summarize the variables with a brief description for the PF and PO records, respectively. Fields and variables of these records are described in detail in Appendix A.

A PF record is placed in the DAT file after a WR or IF record. The optional PO record is placed after a PF record. The computations for the PF/PO record pair are considered in steps 19 and 20 of the target building process as outlined in Chapter 2. Any number of PF/PO record pairs may be assigned to the same WR or IF record.