MATERIAL Y MÉTODO

FLOW ROUTING AND FORECASTING

Routing consists of modeling the timing and attenuation of changes to stream flow. The purpose of forward routing is to adjust stream flows in current and future days at downstream locations for the effects of stream flow depletions, return flows, and other flow changes resulting from water right actions at control points located further upstream. Reverse routing is incorporated in flow forecasting to replicate the effects of forward routing. The purpose of flow forecasting is to allow future days to be considered in determining the volume of stream flow available for water supply and channel capacity available for flood control operations.

Flow routing and forecasting are incorporated in a daily SIMD simulation as outlined in Table 2.4 and summarized in the last section of the preceding Chapter 2. The mechanics of the routing and forecasting techniques are described in the present Chapter 3. Routing and forecasting are integral components of the SIMD water accounting procedures.

Overview of the SIMD Water Accounting Procedures

The SIMD simulation steps through time. At each time step, computations are performed for each water right in priority order. With either a daily or monthly simulation, as each set of water management and use requirements is considered in the water right priority loop, the tasks described in Table 3.1 are performed. Flow forecasting with reverse routing is performed in conjunction with Task 1. Routing of flow adjustments is performed in conjunction with Task 4.

Table 3.1

Computations Repeated for Each Water Right at Each Time Step

______________________________________________________________________________ Task 1: Availability Determination. – The amount of stream flow available to the water

right is the minimum of the control point flow CPFLOW array available flows at the control point of the water right and at relevant control points located downstream, optionally adjusted for channel losses and/or routing. In simulating flood control operations, the amount of channel flood flow capacity below maximum allowable non-damaging limits is determined considering the control point of the flood control right and pertinent downstream control points.

Task 2: Target Set. – The water supply diversion target, hydroelectric power generation target, minimum instream flow limit, or non-damaging flood flow limit is set. Task 3: Water Right Simulation. – For the water right being considered, decisions are made

regarding reservoir storage and releases, water supply diversions, and other water management/use requirements, and appropriate actions are taken. Net evaporation volumes are determined. Water balance accounting computations are performed. Task 4: Flow Adjustment. – The CPFLOW array used to determine water availability and

remaining flood flow capacity in Task 1 is adjusted for the effects of the Task 3 water management and use actions associated with that particular water right.

The CPFLOW control point flow availability array represents available stream flow amounts at the current step in the water right priority-based simulation computations considering each control point location individually. At the beginning of the simulation time step, the

CPFLOW array is populated with naturalized flows plus CI record constant inflows and next-

period return flows from the preceding time step. The CPFLOW array is applied in Task 1 of Table 3.1 to determine stream flow availability for each water right in the priority sequence. In Task 4 in Table 3.1, the amounts in the array are adjusted in the water rights computational loop nested within the time step loop to reflect the impacts of each right. At the end of the simulation time step, the array is used to determine regulated and unappropriated flows.

Flow Forecasting and Routing

Flow forecasting in SIMD is the process of considering future flows over a forecast period in determining water availability for WR record water rights and available flood flow channel capacity for FF/FR record flood control rights. The forecast period set automatically by

SIMD may be replaced by a user-specified forecast period for all or individual water rights.

Routing in SIMD is the process of modeling time lag and storage effects as adjustments to river flows for upstream water control/use actions are propagated downstream over time. Two alternative routing methods are incorporated in SIMD. A lag and attenuation routing method developed specifically for SIMD is the recommended standard default option. An adaptation of the Muskingum routing method is also incorporated in SIMD. Both alternative methods have two parameters representing flow travel time and storage attenuation in a river reach.

The relevance of flow forecasting and routing depends upon the relative magnitude of computational time steps and flow travel times between control point locations. The effects of reservoir operations and other water management and use actions usually propagate through a river/reservoir system in less time than a month. Forecasting and routing are typically not applied in a monthly time step simulation for even very large river systems. Forecasting and routing are typically appropriate for daily simulations of relatively large river systems. With time steps of one-fifth or one-fourth of a month, forecasting and routing may or may not be appropriate depending upon the reach lengths and flow travel times involved in the simulation. The effects of forecasting and routing on simulation results are also affected by the options for setting daily diversion, hydropower, and instream flow targets discussed in Chapter 2.

Routing Adjustments for Next Day Placement of Routed Flow Changes

SIM and SIMD monthly simulation computations always maintain volume balances that

properly account for all inflows, outflows, and changes in storage. However, due to inaccuracies in forecasting and routing, control point flow availability array values may drop below zero in the SIMD computations. SIMD sets negative regulated flows equal to zero and postpones consideration of the necessary amount of routed depletions until the next time step. The routed depletions are applied to regulated flows at the start of in the next time steps until regulated flow meets or exceeds the amount of routed depletions. Adjustment of the timing of routed depletion consideration allows stream flows to remain at or above zero and also maintains the long-term volume balance. Parameter RTGSMM in JT record field 13 activates an option in which monthly totals of routing adjustments are tabulated in the message file on a control point basis.

Options for next-day placement of routed flows controlled by JU record parameters

WRMETH and FRMETH are described in Appendix A. With option 1, the summation of routed

flow adjustments from the preceding day is incorporated in the CPFLOW array at the beginning of the simulation sequence for the current day. Thus, actions of water rights in preceding days may affect stream flow availability in the current day for any water rights including senior rights. With option 2, flow adjustments generated by each individual water right are maintained within the priority sequence. WRMETH option 2 protects senior rights in the current days from stream flow depletions by junior rights during preceding days.

WRMETH option 1 minimizes needs for routing adjustments, but does not protect senior

rights in the current day from actions of junior rights in previous days. WRMETH option 2 protects senior rights but, with imperfect routing and imperfect or no flow forecasting, allows senior rights to take stream flow that has already been depleted by junior rights in previous days. Thus, the potential for making routing adjustments is increased.

Negative Incremental Flow Options

ADJINC in JD record field 8 is a switch for selecting between options associated with the

determination of the amount of stream flow available to a water right in Task 1 of Table 3.1 based on CPFLOW array flows at downstream control points. The alternative ADJINC options are described in the Reference and Users Manuals from the perspective of a monthly SIM simulation. The ADJINC options represent alternative approaches for dealing with the effects of downstream senior rights and negative incremental naturalized flows in checking the CPFLOW array available flows. ADJINC options 2, 3, and 4 activate flow adjustments that deal with negative incrementals. The options differ in the selection of downstream control points to include in the CPFLOW array flow comparison.

Negative Incremental Flows

Naturalized, regulated, and unappropriated flow volumes, and SIM/SIMD algorithms are all based on cumulated total flows at each control point, rather than incremental local flows between control points. However, with a monthly simulation interval (with no routing), the term

negative incremental flow is applied to describe situations in which the naturalized flow volume

for a particular time step at a control point is less than concurrent flows at control points located upstream. Negative incremental means the flow is decreasing in a downstream direction in that time interval. With a monthly time step, by definition, negative incrementals do not exist in a naturalized flow dataset if flows in each time step always increase going downstream.

A daily simulation is complicated by routing which extends the concept of negative incremental flows across multiple time steps. With routing, incremental flows at a particular control point are viewed conceptually as total naturalized flows originating from the current and preceding days routed from one or more (multiple-tributary) adjacent upstream control points less the total naturalized flow at the particular control point. These incremental flows are usually positive but may be negative. The concept of negative incremental flows is fundamental to both daily and monthly simulations even though the computations are based on total flows. Alternative options for dealing with negative incremental flows can significantly affect simulation results in either a monthly or daily model.

Relevant Control Points Considered in the Determination of Available Flow

In Task 1 of Table 3.1, the stream flow available to a water right is determined as the minimum of the CPFLOW array flows at the control point of the right and selected control points located downstream. Without routing and forecasting, only CPFLOW array available flows in the current period are considered. With routing and forecasting, CPFLOW flows in the current day and each day of the forecast are considered. For a particular water right, the set of control points included in determining flow availability includes the control point of the water right and those additional control points that meet all three of the following criteria:

1. located downstream of the control point of the water right

2. identified in the routing and reverse routing as discussed later in this chapter 3. location of senior water rights if either ADJINC option 5, 6, or 7 is activated

Routing and reverse routing determines combinations of future days and control points to be included in the flow availability computations. The third criteria listed above for selecting downstream control points is applicable only if JD record ADJINC option 5, 6, or 7 is selected.

The amount of stream flow available to a water right in Task 1 of Table 3.1 is the minimum CPFLOW array available flows in the current and forecast days at the control point of the water right and selected downstream control points. Flow at downstream control points may be the minimum in the CPFLOW array comparison and thus limit the amount of flow available to the water right located upstream only if one or more of the following conditions occur:

1. junior rights decrease the flows at one or more of the downstream control points 2. senior rights decrease the flows at one or more of the downstream control points 3. negative incremental flow situations affect the flow availability computations

The purpose of forecasting is to prevent junior rights from reducing the stream flow available to senior water rights in future days. Therefore, with forecasting activated, the above list of factors affecting flow available to a particular right is reduced to the effects of senior rights and negative incremental flows. Thus, ADJINC options 5, 6, and 7 limit the search for the constraining minimum CPFLOW flow to the control points of the water right and downstream senior rights.

Options Activated by ADJINC in JD Record Field 8

Negative incremental flow options 1, 2, 3, 4, and 5 date back to early versions of the monthly SIM. Option 1 considers all downstream control points in selecting the minimum flow quantity from the CPFLOW array and applies no incremental flow adjustments. Options 2 and 3 are seldom if ever used. Option 4 has been the recommended standard for a monthly simulation. Option 5 is also commonly used for monthly simulations, but is the only ADJINC option that cannot be activated with a daily simulation.

As explained in Chapter 3 of the Reference Manual, options 4 and 6 involve a flow adjustment defined as the minimum amount of flow that must be added to the naturalized flow at a control point to alleviate all negative incremental naturalized. SIMD computes and applies negative incremental flow adjustments for a daily time step in the same manner as the monthly

SIM. SIMD first determines daily naturalized flows at all control points and then computes daily

adjustments in the same manner that SIM applies monthly adjustments. In determining stream flow available for WR record water rights and filling FR record flood control reservoir storage at a particular control point, the adjustment amounts are added to control point flows at downstream control points but not at the control point of the water right. Since the negative incremental flows are defined by concurrent upstream and downstream flows in the same time step, options 4 and 6 are generally not applicable for a daily simulation that includes routing.

The new ADJINC options 6, 7, and 8 are defined as follows. Relevant senior rights are those that appropriate stream flow, which excludes types 3, 4, and 6 (WR record field 6).

Option 6 is same as option 4 except the downstream control points used in selecting the minimum flow from the CPFLOW array are limited to the sites of relevant senior rights. Option 7 is same as option 1 except the downstream control points used in selecting the minimum flow from the CPFLOW array are limited to the sites of relevant senior rights. Option 8 incorrectly ignores all downstream control points. The CPFLOW array flow at the control point of the water right is assumed to be the flow available to the water right.

Any of the eight ADJINC options can be adopted in SIM or SIMD monthly or daily simulations, except option 5 is not allowed in a daily simulation. Option 7 is recommended whenever routing is adopted, which is typically the case in a daily simulation. Option 1 restricts the flow amount available to water rights more than option 7 but is also applicable with routing. Option 6 is recommended if routing is not activated.

Option 7 is designed to be the standard ADJINC option to be adopted whenever routing and forecasting are employed, but can also be used in a monthly or daily simulation without routing and forecasting. The downstream control points identified in the reverse routing are further constrained to only those control points at which relevant senior rights are located. Flows at downstream control points not affected by senior rights have no effect on water availability for the junior right. Therefore, negative incremental flows at a downstream control point affect the amount of flow available to a particular water right only if senior rights also reduce the flows at the downstream control point. Option 7 is similar to option 5 but does not include all features of option 5. Option 7 is option 1 with the limitation to senior right control points added.

Option 6 is identical to 4 except only the control points with senior rights are considered. Options 6 and 4 should yield essentially the same simulation results though option 4 is more conservative in assuring that flow cannot be over-appropriated. Option 8 allows investigation of the effects of junior rights not passing inflows to protect downstream senior rights.

Routing Changes to Flow

Routing in SIMD propagates flow changes through river reaches connecting control points. Water supply diversions and return flows and reservoir releases and storage refilling at a control point result in changes in stream flows at downstream control points. Routing in SIMD refers to the downstream propagation of changes resulting from an upstream change to stream flow. Reverse routing replicates the effects of routing in the procedure for forecasting flow availability for WR and IF record rights as explained later in this chapter in the section on forecasting.

In the monthly time step SIM, for a river reach without reservoir storage, outflow volume in a month equals the inflow in the month less channel losses. Likewise, without activation of the routing methods described here, in a daily SIMD simulation, outflow volume from a river reach in a day equals its inflow less channel losses. Routing simulates the storage effects (lag and attenuation) of a river reach on the relative timing of reach outflows and inflows.

Routing Flow Changes Associated with Water Rights

A reach refers to the segment of river between two control points. Routing parameters are entered on the RT record for the control point defining the upstream end of a river reach. Different parameter values may be entered for flow changes associated with flood control FR record reservoir operations and flow changes for WR record rights. If routing parameters are assigned for a control point, routing computations are performed resulting in lag and attenuation of flow changes originating at or passing through the control point. If routing parameters are not specified for a particular control point, flow changes originating at or passing through the control point are passed through the reach below the control point by simple translation without routing computations and thus without lag or attenuation. Without routing, outflow from a river reach in a time step equals the inflow in the time step less channel losses.

Channel losses are computed in both monthly SIM and daily SIMD simulations in both Tasks 1 and 4 of Table 3.1. In Task 4, routing computations are performed after the channel loss computations. The routed flow changes are then further adjusted for channel losses. SIMD routing in Task 4 is replicated as reverse routing in Task 1.

Routing occurs at a control point if and only if routing parameters are specified as input data for that control point. Routing computations normally simulate flow attenuation and lag in the river reach below the specified control point. However, the model user may chose to lump attenuation/lag effects in multiple reaches in routing computations at a single control point. The model user selects the control points at which routing is to be applied. In applications with significant flow travel times between control points, routing parameters may be provided for all control points, except the basin outlet. However, a SIMD model may include control points defining river reaches that are too short to meaningfully apply routing in a daily time step model. The larger river basins in the Texas WAM System have hundreds of control points, many of which are too closely spaced for meaningful routing. For complex datasets with numerous