Contents preface (VII) introduction 1—37

Table 3.5 Indicative values of crop water needs and sensitivity to drought (5)

Crop	Crop water need	Sensitivity of drought
(mm/total growing period)
Alfalfa	800	-	1600	low	-	medium
Banana	1200	-	2200	high
Barley/oats/wheat	450	-	650	low	-	medium
Bean	300	-	500	medium	-	high
Cabbage	350	-	500	medium	-	high
Citrus	900	-	1200	low	-	medium
Cotton	700	-	1300	low
Maize	500	-	800	medium	-	high
Melon	400	-	600	medium	-	high
Onion	350	-	550	medium	-	high
Peanut	500	-	700	low	-	medium
Pea	350	-	500	medium	-	high
Pepper	600	-	900	medium	-	high
Potato	500	-	700	high
Rice (paddy)	450	-	700	high
Sorghum/millet	450	-	650	low
Soybean	450	-	700	low	-	medium
Sugarbeet	550	-	750	low	-	medium
Sugarcane	1500	-	2500	high
Sunflower	600	-	1000	low	-	medium
Tomato	400	-	800	medium	-	high

108 IRRIGATION AND WATER RESOURCES ENGINEERING
Example 3.6 Using the Blaney-Criddle formula, estimate the yearly consumptive use of water for sugarcane for the data given in the first four columns of Table 3.6.
Solution:
According to Eqs. (3.13) and (3.14),
u = k ₁₀₀^p (18. t + 32)
Values of monthly consumptive use calculated from the above formula have been tabulated in the last column of Table 3.6. Thus, yearly consumptive use = Σu = 1.75 m.
Table 3.6 Data and solution for Example 3.6

	Mean monthly	Monthly crop	Per cent	Monthly
Month	temperature,	coefficient, k	sunshine	consumptive
	t°C		hours, p	use, u (mm)
January	13.10	19.05	7.38	78.14
February	15.70	20.32	7.02	85.96
March	20.70	21.59	8.39	125.46
April	27.00	21.59	8.69	151.22
May	31.10	22.86	9.48	190.66
June	33.50	24.13	9.41	209.58
July	30.60	25.40	9.60	212.34
August	29.00	25.40	9.60	205.31
September	28.20	24.13	8.33	166.35
October	24.70	22.86	8.01	140.01
November	18.80	21.59	7.25	103.06
December	13.70	19.05	7.24	78.15

3.8. IRRIGATION REQUIREMENT
Based on the consumptive use, the growth of all plants can be divided into three stages, viz., vegetative, flowering, and fruiting. The consumptive use continuously increases during the vegetative stage and attains the peak value around the flowering stage; thereafter, the consumptive use decreases. It should be noted that different crops are harvested during different stages of crop growth. For example, leafy vegetables are harvested during the vegetative stage and flowers are harvested during the flowering stage. Most crops (such as potatoes, rice, corn, beans, bananas, etc.) are harvested during the fruiting stage.
At each precipitation, a certain volume of water is added to the crop field. Not all of the rainfall can be stored within the root zone of the soil. The part of the precipitation which has gone as surface runoff, percolated deep into the ground or evaporated back to the atomosphere does not contribute to the available soil moisture for the growth of crop. Thus, effective precipitation is only that part of the precipitation which contributes to the soil moisture available for plants. In other words, the effective rainfall is the water retained in the root zone and is obtained by subtracting the sum of runoff, evaporation, and deep percolation from the total rainfall.

SOIL-WATER RELATIONS AND IRRIGATION METHODS

109

If, for a given period, the consumptive use exceeds the effective precipitation, the difference has to be met by irrigation water. In some cases irrigation water has to satisfy leaching requirements too. Further, some of the water applied to the field necessarily flows away as surface runoff and/or percolates deep into the ground and/or evaporates to the atmosphere. Therefore, irrigation requirement is the quantity of water, exclusive of precipitation and regardless of its source, required by a crop or diversified pattern of crops in a given period of time of their normal growth under field conditions. It includes evapotranspiration not met by effective precipitation and other economically unavoidable losses such as surface runoff and deep percolation. Irregular land surfaces, compact impervious soils or shallow soils over a gravel stratum of high permeability, small or too large irrigation streams, absence of an attendant during irrigation, long irrigation runs, improper land preparation, steep ground slopes and such other factors contribute to large losses of irrigation water which, in turn, reduce irrigation efficiency. Irrigation efficiency is the ratio of irrigation water consumed by crops of an irrigated field to the water diverted from the source of supply. Irrigation efficiency is usually measured at the field entrance (3). Water application efficiency is the ratio of the average depth added to the root-zone storage to the average depth applied to the field. Obviously, irrigation efficiency measured at the field and the water application efficiency would be the same. Thus, the field irrigation requirement FIR is expressed as (2)

	Det − (D p	− Dpl )
FIR =			(3.15)
Ea

in which, D_et = depth of evapotranspiration, D_p = depth of precipitation,
D_pl = depth of precipitation that goes as surface runoff and/or infiltrates into the ground and/or intercepted by the plants,
and E_a = irrigation efficiency or application efficiency.
In the absence of any other information, the following values can be used as a guide for E_a in different methods of surface irrigation for different types of soils:

Soil class	Irrigation efficiency (%)
Sand	60
Sandy loam	65
Loam	70
Clay loam	75
Heavy clay	80

If no other information is available, the following formulae can be used to estimate the effective rainfall depth, D_pe provided that the ground slope does not exceed 5%.

D_pe = 0.8 D_p – 25 if D_p > 75 mm/month

D_pe = 0.6 D_p – 10 if D_p < 75 mm/month

D_pe is always equal to or greater than zero and never negative. Both D_p ad D_pe are in mm/month in the foregoing formulae.
Example 3.7 Using the data given in the first four columns of Table 3.7 for a given crop, determine the field irrigation requirement for each month assuming irrigation efficiency to be 60 per cent.

110					IRRIGATION AND WATER RESOURCES ENGINEERING