Comparison of Consumptive Water Use Models and Methods for Drought Water Requirements in Florida

In Florida, the models and application methods used to issue consumptive use permits (CUPs) generate regionally disparate crop water requirements for the same crops and challenge the equitable distribution of water among farmers. Three Water Management Districts (WMDs), the St. Johns River WMD (SJRWMD), the South Florida WMD (SFWMD) and the Southwest Florida WMD (SWFWMD) issue the majority of the CUPs. Their CUP models are the Agricultural Field Scale Irrigation Requirements Simulation v5.5 (AFSIRS) model, the SCS TR-21 method developed by the Soil Conservation Service (SCS), and the Agricultural Water Use Model v2.0 (AGMOD). An analysis was performed to identify the sources and magnitudes of these differences among the three models and the WMDs' application methods. The quantitative analysis compared the annual and monthly drought water demands from three models for a citrus grove and a pasture crop at a hypothetical farm located at the convergence of the three WMDs. When applied using the WMDs' methodologies, the models yielded 5-yr drought net irrigation requirements that ranged from 36 to 60 cm for citrus and from 41 to 48 cm for pasture. Differences among the modeled crop evapotranspiration (ET) losses, 26 and 18 cm for citrus and pasture, respectively, were the major source of the net irrigation variability. The relatively small effective rainfall differences, 7 cm for citrus and 10 cm for pasture, offset the ET loss disparity and reduced the drought net irrigation values. Application of the models with identical climate data and soil parameters decreased the net irrigation variability for citrus, but increased it slightly for pasture. Again the modeled ET was found to be the primary source of the differences.