**3.4 Irrigation water requirement**

The irrigation water requirement represents the difference between the crop water requirement and effective rainfall. Other factors or losses have minimal effect on irrigation water requirement and can be neglected [23] as shown in the equation below:

$$\text{IR} = \text{ET}\_{\text{c}} \text{--} (\text{P}\_{\text{e}} + \text{G}\_{\text{e}} + \text{W}\_{\text{b}}) \tag{3}$$

D = Depth of root development (mm).

*DOI: http://dx.doi.org/10.5772/intechopen.94076*

**4.1 Management allowable depletion (MAD)**

be determined in the following form:

Where,

affect harmful way.

**CROPWAT model**

hydraulic conductivity was 0.77 cm h�<sup>1</sup>

**17**

Bd = Bulk density of the particular soil layer (g cm�<sup>3</sup>

θTSMC: Soil moisture content at threshold level (%). θFC: Soil moisture content at field capacity (%).

θPWP: Soil moisture content at permanent wilting point (%).

**5. Estimation of water requirement of major crops using**

The determination of soil moisture content at threshold level is most important factor for irrigation scheduling on real time basis. This value varies with crop, soil, climate and crop growth stages. Whenever the soil moisture content at field capacity is depleted through ET, percolation losses, etc. to equal or below the θTSMC value, irrigation scheduling must be given otherwise crop yield and plant growth will be

CROPWAT is a decision support tool developed by the land and water development division of FAO. CROPWAT model is extensively tested, widely accepted for calculation of crop water requirements based on soil, climate and crop data. In addition, the program allows the development of irrigation schedules for different management and the calculation of scheme water supply for varying crop patterns. CROPWAT ver. 8.0 model used weather data and employs the modified penmanmonteith approach used to estimate reference evapotranspiration on a daily basis. The meteorological data was taken from Agromet Observatory, ICAR-VPKAS, Experimental farm Hawalbagh, Almora. The mean annual rainfall at experimental site was 1000.13 mm. The general soil properties of the experimental field were used in CROPWAT model. Based on the details of soil characteristics, total available water was taken 135 mm m�<sup>1</sup> depth of soil. Infiltration rate was measured using double ring infiltrometer and the basic rate was 6.8 mm hr.�<sup>1</sup> and the unsaturated

ment (346–376 mm, 131–189 mm, 1.4 mm, 1.3 mm, 78.6 mm, 93.5 mm 104.1 mm, 176 mm, 96.9 mm, 16.2 mm, 18.3 mm, 15.5 mm, and 7.4 mm of Rice, Wheat, Maize, Soybean, Vegetable Pea, Rajma, Barley, Tomato, French Bean, chili, okra, mustard

Producing optimal yield requires that the soil water content be maintained between an upper limit at which leaching becomes excessive and a lower point at which crops are stressed [27]. As water is removed from the soil through ET, there is a point below which the plant experiences increasing water stress. This point is known as the management allowable depletion (*MAD*). The typical MAD values considered are 33% for shallow-rooted, high value crops; 50% for medium-rooted, moderate value crops and 67% for deep-rooted, low value crops [28]. Selection of MAD value for different crops with respect to soil type, initial field capacity (FC), permanent wilting point (PWP), and threshold soil moisture content (TSMC) must be determined. Threshold soil moisture content ascertains what fraction of soil is allowed to dry before the next irrigation event. Threshold soil moisture content can

*Climatic Variation and Its Impacts on Yield and Water Requirement of Crops in Indian Central…*

MAD = Management allowable Depletion (%).

).

θTSMC ¼ θFC–MAD ð Þ θFC–θPWP (5)

. The estimation of irrigation water require-

Where,

IR = irrigation requirement (mm). Etc = total crop evapotranspiration (mm). Pe = effective rainfall (mm). Ge = groundwater contribution from water table (mm). Wb = water stored in the soil at the beginning of each period (mm).
