Models in Irrigation andWater Management - Practices of Irrigation and On-farm Water Management - page 393

It is expressed as

O i − S i
O i

CRM = (10.7)

(viii) Coefficient of determination (CD):
It is expressed as

( O i −

O ) 2

CD = (10.8)

( S i −

O ) 2

10.3 Overview of Some of the Commonly Used Models

In recent years, mathematical modeling and simulation techniques, relying on the
use of high speed computer, have been developed for the purpose of providing com-
prehensive quantitative predictions. The application of system analysis (modeling)
techniques to agricultural cropping systems is now widespread. A number of models
for scheduling irrigations under limited water supplies have been developed. Many
models determine the optimal irrigation strategies using stochastic or probabilistic
methods of weather variables (Mapp and Ediman, 1987 ; Rao et al., 1988 ; Rao et al.,
1990 ) . But in any season, the current weather variables can be significantly different
from their probabilistic or stochastic estimates.
Some of the commonly used models are outlined here for general orientation.

10.3.1 Model for Reference Evapotranspiration (ET 0 Models)

Models for predicting reference evapotranspiration (ET 0 ) range from determinis-
tically based combined energy balance-vapor transfer approaches to empirically
relationships based on climatological variables or to evaporation from a standard
evaporation pan. Updated procedures for calculating ET 0 were established by FAO
(Smith et al., 1992).

10.3.1.1 CROPWAT Model

The FAO developed CROPWAT software (Clarke, 1998 ) to calculate ET 0 and irriga-
tion planning. It is available both in DOS and Windows version. The ET 0 estimation
is based on Penman-Monteith (P-M) equation. The inputs for ET 0 estimation are (i)
maximum temperature, (ii) minimum temperature, (iii) average humidity, (iv) wind
speed, (v) sunshine hour, (vi) latitude, and (vii) altitude of the location. The original
form of the Penman-Monteith equation (Monteith, 1981) can be written as