A farm package for MODFLOW-2000 : simulation of irrigation demand and conjunctively managed surface-water and ground-water supply

Abstract

A new Farm Package (FMP) was developed for using the U.S. Geological Survey's groundwater modeling program, MODFLOW-2000 (MF2K), to estimate irrigation water allocations to irrigation settings. The FMP dynamically integrates irrigation water demand, surface-water & groundwater supply, and return flow from excess irrigation. Routed surface-water delivery is optional, and can be simulated by coupling FMP with the Streamflow Routing Package (SFR1). MF2K with FMP and SFR1 allows estimating the allocation of surface-water and groundwater to farms for the following applications: (1) historic and future simulations, (2) water rights issues and operational decisions, (3) non-drought and drought situations. Irrigation demand, supply, and return flow are partly subject to head-dependent sinks and sources such as transpiration uptake from groundwater (formulated by FMP) and leakage between the conveyance system and the aquifer (formulated by SFR1). A steady state transpiration uptake, varying with changing water level, is stepwise linearly approximated by FMP. This was validated by ensembles of variably saturated soil column models using HYDRUS2D for different soil types, values of potential transpiration, and root zone depths. A restriction of transpiration uptake is proportional to a reduction of the active root zone. It is approximated in FMP by an analytical solution, which determines inactive ranges of the root zone with pressure heads typical for conditions of anoxia or wilting. At steady state, the transpiration uptake equaled the flux across the water table (plus the irrigation flux, if applied). Therefore, changes in soil water storage are assumed negligible. Based on this assumption, the irrigation flux required is determined in FMP by subtracting transpiratory components from natural sources (groundwater, precipitation) from a maximum transpiration uptake. This transpiratory irrigation requirement is calculated for each finite difference cell, and increased sufficiently to compensate for evaporative losses and for inefficient use. Accumulating the resulting cell delivery requirement over all cells in a farm yields the total farm delivery requirement, which is to be satisfied with surface- or groundwater. Five economic and non-economic drought response policies can be applied, if the potential supply of surface- and groundwater is insufficient to meet the crop demand. The code was verified by a hypothetical example problem run in 55 scenarios (5 drought policy scenarios x 11 parameter-group scenarios). Among all sources and sinks in a cumulative volumetric budget, 'farm well discharge,' and particularly 'farm net recharge,' were most sensitive to changes in drought policies or changes of parameters.