Contents preface (VII) introduction 1—37

Example 3.4 For the following data, calculate the total available water and the soil moisture defict.

Soil depth (cm)	Gb	^Wfc	^Wwp	W
0-15	1.25	0.24	0.13	0.16
15-30	1.30	0.28	0.14	0.18
30-60	1.35	0.31	0.15	0.23
60-90	1.40	0.33	0.15	0.26
90-120	1.40	0.31	0.14	0.28

102			IRRIGATION AND WATER RESOURCES ENGINEERING
	Solution:
	Depth of soil	^wfc ^= ^Gb ^Wfc	^wwp ^= ^Gb ^Wwp	dt = d ×	w = Gb W	Ds = d ×
	layers, d			(wfc – wwp)		(wfc – w)
	(mm)			(mm)		(mm)
	150	0.3	0.1625	20.625	0.2	15.0
	150	0.364	0.182	27.300	0.234	19.5
	300	0.4185	0.2025	64.800	0.3105	32.4
	300	0.462	0.21	75.600	0.364	29.4
	300	0.434	0.196	71.400	0.392	12.6
			Total	259.725		108.9

Example 3.5 The field capacity and permanent wilting point for a given 0.8 m root-zone soil are 35 and 10 per cent, respectively. At a given time, the soil moisture in the given soil is 20 per cent when a farmer irrigates the soil with 250 mm depth of water. Assuming bulk specific gravity of the soil as 1.6, determine the amount of water wasted from the consideration of irrigation.
Solution:
At the time of application of water,
Soil moisture deficit, D_s = (W_fc – W) d G_b

• 
  (0.35 – 0.20) (0.8) (1.6)
  

• 
  0.192 m
  

Therefore, the amount of water wasted

• 
  0.250 – 0.192
  

• 
  58 mm
  

• 
  ₂₅₀⁵⁸ × 100 = 23.2%
  

3.6. INFILTRATION
Infiltration is another important property of soil which affects surface irrigation. It not only controls the amount of water entering the soil but also the overland flow. Infiltration is a complex process which depends on: (i) soil properties, (ii) initial soil moisture content, (iii) previous wetting history, (iv) permeability and its changes due to surface water movement, (v) cultivation practices, (vi) type of crop being sown, and (vii) climatic effects. In an initially dry soil, the infiltration rate is high at the beginning of rain (or irrigation), but rapidly decreases with time until a fairly steady state infiltration is reached (Fig. 3.3). This constant rate of infiltration is also termed the basic infiltration rate and is approximately equal to the permeability of the saturated soil.
The moisture profile under ponded infiltration into dry soil, Fig. 3.4, can be divided into the following five zones (4):

SOIL-WATER RELATIONS AND IRRIGATION METHODS	103
10							50
cm/hr								Zincm
IInfiltrationrate,in							Cumulativeinfiltration,
0
0	Infiltration time, hours	20

Fig. 3.3 Variation of infiltration rate, I and cumulative infiltration, Z with time

O	θi	Water content
				Saturated zone
Soildepth		Initialwatercontent		Transition zone
			Transmission
				zone
				Wetting zone
				Wetting
				front

θ_s

Saturatedwatercontent

Fig. 3.4 Soil-moisture profile during ponded infiltration
(i) The saturated zone extending up to about 1.5 cm below the surface and having a saturated water content.
(ii) The transition zone which is about 5 cm thick and is located below the saturated zone. In this zone, a rapid decrease in water content occurs.
(iii) The transmission zone in which the water content varies slowly with depth as well as time.
(iv) The wetting zone in which sharp decrease in water content is observed.
(v) The wetting front is a region of very steep moisture gradient. This represents the limit of moisture penetration into the soil.
Table 3.1 lists the ranges of porosity, field capacity, permanent wilting point, and basic infiltration rate (or permeability) for different soil textures.

104			IRRIGATION AND WATER RESOURCES ENGINEERING
		Table 3.1 Representative properties of soil
	Soil texture	Porosity (%)	Field capacity	Permanent	Basic infiltration
			(%)	wilting point (%)	rate (cm/hr)
	Sand	32-42	5-10	2-6	2.5-25
	Sandy loam	40-47	10-18	4-10	1.3-7.6
	Loam	43-49	18-25	8-14	0.8-2.0
	Clay loam	47-51	24-32	11-16	0.25-1.5
	Silty clay	49-53	27-35	13-17	0.03-0.5
	Clay	51-55	32-40	15-22	0.01-0.1