Implementing the dual crop coefficient approach in interactive software. 1. Background and computational strategy

Irrigation planning and scheduling require the availability of modeling tools that are accurate, quick and easy to use. The crop coefficient (Kc)-reference evapotranspiration (ET) method is a traditional method for estimating ET, but has become relatively complicated with the introduction of the dual Kc procedure. The dual crop coefficient approach (Kcb + Ke) gives a better estimation of daily crop evapotranspiration because it separately considers soil evaporation and crop transpiration. This approach allows one to plan irrigation schedules properly, especially in the case of crops that do not completely cover the soil, where evaporation from the soil surface may be substantial. The SIMDualKc software application was developed with the purpose of simplifying implementation of the computation of the crop coefficient and crop evapotranspiration using the dual crop coefficient approach over a range of cultural practices and to provide ET information for use in irrigation scheduling and hydrologic water balances. The model performs a soil water balance at the field level using a daily time step. It estimates crop transpiration and soil evaporation as well as soil water dynamics to support irrigation scheduling for full and incomplete cover crops. This paper is the first part of a two-part series, where the second part describes model testing and application for various crops, locations and irrigation management issues.

► This paper presents a water balance simulation model that uses the dual crop coefficient approach to estimate the vegetation evapotranspiration, thus separating transpiration from soil evaporation; model testing is presented in a second part paper.
► For incomplete cover crops, the basal crop coefficient is estimated from the fraction of ground cover through a density factor, and active ground cover may also be considered.
► Soil evaporation is computed considering the fraction of wetted soil exposed to radiation and the effects of organic or plastic mulch.
► Deep percolation and groundwater contribution may be computed with a parametric function; runoff of excess precipitation is considered through the curve number approach.