Models in Irrigation andWater Management - Practices of Irrigation and On-farm Water Management - page 402

where

z = change in compartment thickness

α = soil constant/gradient; different for different k ( h ) line segment

10.3.3.2 MACRO

The MACRO model is a comprehensive model of both the field water balance and
solute transport and transformation processes in the soil/crop system (Jarvis, 1994 ) .
An important feature of the model is that it may be run in either one or two flow
domains. In two domains, the total porosity is partitioned into macropores and
micropores. Each domain is characterized by a degree of saturation, conductivity,
and a flux, while interaction terms account for convective and diffuse exchange
between flow domains. In one domain, the interaction terms are redundant and
the model simply reduces to standard numerical solutions of the Richard's and
convection-dispersion equations.

Soil-Water Balance

Water movement in the micropores is calculated with the Richard's equation includ-
ing a sink term to account for root water uptake (Jarvis, 1994 ) . The soil hydraulic
properties are described by the functions of Brooks and Corey and Mualem. Water
flow in macropores is calculated with Darcy's law assuming a unit hydraulic gra-
dient and a simple power law function to represent the unsaturated hydraulic
conductivity. Driving variables in the model consists of measured rainfall data at
a given solute concentration, daily potential evapotranspiration and, if pesticide
transport is being considered, daily maximum and minimum air temperatures.
The surface boundary condition in MACRO partitions the net rainfall into an
amount taken up by micropores and an excess amount of water flowing into macro-
pores. This partitioning is determined by a simple description of the infiltration
capacity of the micropores. Alternative options for the bottom boundary condition
are available in the model.
Root water uptake is predicted as a function of the evaporative demand, root
distribution and soil water content using a simple empirical model. The calculated
water uptake is assumed to be preferentially satisfied from the water stored in the
macropores.

10.3.4 Model for Irrigation Planning and Decision Support System

10.3.4.1 CROPWAT

CropWat for Windows is a program that uses the FAO Penman-Monteith method
(Smith et al., 1992) for calculating reference crop evapotranspiration. These esti-
mates are used in crop water requirements and irrigation scheduling calculations