Hydrus Model

An important modelling approach, aimed to simulate water and solute transport through the soil vadose zone is the model HYDRUS-1D (Simunek et al., 2005). The model consists of the HYDRUS computer program, and the HYDRUS 1D interactive graphics-based user interface. According to Simunek et al. (2005), the HYDRUS program numerically solves the Richards equation for saturated-unsaturated water flow and convection-dispersion type equations for heat and solute transport. The water flow equation incorporates a sink term to account for water uptake by plant roots, as well as in SWAP (see equation 1). The heat transport equation considers movement by conduction as well as convection with flowing water. Salinity is also considered through the Maas and Hoffman (1977) function and the simulation of crop water uptake follows a similar approach than the SWAP model.

The HYDRUS-1D code may be used to analyze water and solute movement in unsaturated, partially saturated, or fully saturated porous media. The flow region itself may be composed of non-uniform soils (Simunek et al., 2005). Flow and transport can occur in the vertical, horizontal, or in a generally inclined direction. The water flow part of the model considers prescribed head and flux boundaries, as well as boundaries controlled by atmospheric conditions, free drainage, or flow to horizontal drains (Simunek et al., 2005).

The source code was developed and tested on a Pentium 4 PC using the Microsoft's Fortran PowerStation compiler. Several extensions of the MS Fortran beyond the ANSI standard were used to enable communication with graphic based user-friendly interface (Simunek et al., 2005). HYDRUS1D comprises an interactive graphics-based user-friendly interface for the MS Windows environment. The HYDRUS1D interface is directly connected to the HYDRUS computational programs. Besides, the HYDRUS program come with several utility programs that make easier the data input process.

HYDRUS1D can be considered as a one-dimensional version of the HYDRUS-2D code. This updated modelling release is aimed to simulate water, heat and solute movement in two-dimensional variably saturated media (Simunek et al., 1998), while incorporating various features of earlier related codes such as SUMATRA (van Genuchten, 1978), WORM (Van Genuchten, 1987), HYDRUS 3.0 (Kool and van Genuchten, 1991), SWMI_ST (Simunek, 1992), and HYDRUS 5.0 (Vogel et al., 1996). Indeed, to be able to simulate soil water-movement and crop water-uptake in two dimensions open new possibilities that perhaps make HYDRUS2D the most important currently available model in this concern (Van Genuchten and Simunek, 2005).

3.2.5 Agrohydrological Models Constraints

Many models comparisons have pointed out that despite mechanistic model yield more accurate and sounder simulations; they require many input parameters that are not always easy to measure or to estimate (Leenhardt et al., 1995; Connolly, 1998; Bastiaansen et al., 2004). Sensitivity analysis of SWAP outputs showed that the simulated water balance are most sensitive to the crop coefficients used for calculating potential transpiration and to the soil hydraulic properties (Van Dam, 2000). This agrees with other analysis made with similar agrohydrological modelling approaches, based on Richards' equation (Clemente et al., 1994; Leenhardt et al., 1995). Unfortunately, the soil hydraulic properties use to show high spatial variability (Warrick and Nielsen, 1980; Van Genuchten, 1994; Leenhardt et al., 1995). Therefore, the SWAP dependence on these properties can be seen as one of the highest constraints when using such modelling approach (Van Genuchten, 1994; Leenhardt et al., 1995; Van Dam, 2000).

3.2.6 Simple Crop Water-Use Simulation Models. CROPWAT

Very often availability of model input data (especially soil input data) is a serious limitation for applications of complex crop water balance models as to be used for irrigation scheduling. This is especially a problem in poor agricultural regions, where input data generation might be a serious cost factor. The limitation on input data quality can lead to the situation that simple models or methods may perform better than the complex models. That is why for all SIRDEM experimental sites the performance of a simple approach will be tested as an alternative option. The test will result in defining limitations and potentials of both simple and complex approaches.

CROPWAT is a free-distributed code from the Food and Agriculture Organization (FAO) and can be downloaded from the FAO web

(http://www.fao.org/landandwater/aglw/cropwat.stm). It is meant as a practical tool to help agro-meteorologists, agronomists and irrigation engineers to carry out standard calculations for evapotranspiration and crop water use studies, and more specifically the design and management of irrigation schemes. CROPWAT allows the development of recommendations for improved irrigation practices, the planning of irrigation schedules under varying water supply conditions, and the assessment of production under rainfed conditions or deficit irrigation. It simulates crop water-requirements on a monthly, 10-days period or daily basis with a simplified soil water balance (cascade approach). Actually, the cascade approach can yield same results than the physically-based approach, based on numerical solutions of Richards's equation, as shown by Eitzinger et al. (2004).

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