The water module contains the following calculation routines:

• Crop evapotranspiration (soil evaporation and transpiration)
• Effective water infiltration (applied water minus runoff)
• Drainage
• Water redistribution in soil

Crop evapotranspiration is calculated using the FAO approach (Allen et al., 1998). The main parameters are those related to the evaporative demand of the atmosphere, summarized by the reference evapotranspiration (ETo,) and a crop coefficient that varies with crop development. The effects of water stress on plant growth are based on the assumption that the reduction in dry matter accumulation due to water deficit is proportional to the transpiration reduction (Hanks, 1983; Shani and Dudley, 2001).

Water infiltration and redistribution in soil follows a capacitance approach, similar to the one in the N_ABLE model but which has been modified using a drainage coefficient that allows the water transfer between layers above field capacity to be done progressively (in more than one day) and more or less rapidly depending on soil type (Ritchie, 1998). Drainage at any depth is given as the water downward flow of the cell elements at this depth. The module also accounts for upward/downward capillary flow by adopting a soil water normalised diffusion approach (Rose, 1968 and Ritchie, 1998). The main parameters that define the hydraulic soil properties such as the water content at field capacity and wilting point, are input by the user for the different soil layers, although default values depending on soil texture are available.

Runoff is calculated using the approach by the U.S. National Resource Conservation Service (NRCS, formerly the Soil Conservation Service) based on studies of small (< 800 ha) agricultural watersheds across the United States.

Literature

Allen R.G., Pereira L.S., Raes D., Smith M.(1998). Crop evapotranspiration. Guidelines for computing crop water requirements. FAO Irrigation and Drainage Paper n° 56, FAO, Roma.

Haan, C.T., B.J. Barfield and J.C. Hayes (1994) Design hydrology and sedimentation for small catchments, Academic Press, New York, 588p.

Hanks R.J. 1983. Yield and water-use relationships: An overview. In H.M. Taylor et al. (ed.) Limitations to efficient water use in crop production. ASA,

Madison, WI., pp. 393-411.

Ritchie, J.T. (1998). Soil water balance and plant water stress. G.Y. Tsuji et al. (eds): Understanding options for agricultural production p. 41-54.

Rose, D. (1968). Water movement in porous materials III. Evaporation of water from soil. Brit. J. Appl. Phys. ser 2(1): 1779-1791.

Shani U. and L. M. Dudley. 2001. Field studies of crop response to water and salt stress. Soil Sci. Soc. Am. J. 65:1522–1528.