Water entering the soil at the surface is called infiltration. It replenishes the soil-moisture deficiency and the excess moves downward by the force of gravity called deep seepage or percolation and builds up the ground-water table. The maximum rate at which the soil in any given condition is capable of absorbing water is called its infiltration capacity. Infiltration often begins at a high rate (20 to 25 cm/hr) and decreases to a fairly steady rate as the rain continues, called the ultimate fp (1.25 cm/hr), Fig. 2.15.
The infiltration rate f at any time t is given by the Horton’s equation-
Where f0 = initial rate of infiltration capacity; fc = final constant rate of infiltration (at saturation); k = a constant depending primarily upon soil and vegetation; e = base of the natural logarithm and Fc = shaded area in Fig. 2.15.
Infiltration depends upon the intensity and duration of rainfall, weather soil characteristics, vegetal cover, land use, initial soil moisture content, entrapped air and depth of water table.
Infiltration data can be obtained as follows:
(i) By analysis of rainfall and runoff records of small drainage basins with homogeneous soils.
(ii) By establishing plots or watersheds and operating them for several years to obtain sufficient data.
(iii) Use of infiltrometers or rate simulators by which water can be applied artificially to small areas.
A double ring infiltrometer is shown in Fig. 2.16 with two feeding tanks to maintain a constant level of water inside the infiltrometer rings. The two rings are driven into the ground by a driving plate and hammer, to penetrate into the soil uniformly without tilt or undue disturbance of the soil surface, to a depth of 15 cm.
After the driving is over, any disturbed soil adjacent to the sides is tamped with a metal tamper. Two point gauges are fixed in the centre of the rings and in the annular space between the two rings.
A minimum water level of 2.5 cm up to a maximum of 15 cm is usually maintained. The water added to maintain the desired depth after the start of the experiment is noted at regular time intervals, say, 5, 10, 15, 20, 30, 40, 60 min up to a period of at least 6 hours and the results plotted as infiltration in mm versus time. A typical infiltration rate curve is shown in Fig. 2.17.
(iv) Observation from infiltration pits and ponds and deducting the loss due to evaporation.
(v) Placing a catch basin called a lysimeter under a laboratory sample or at some depth below the land surface to measure the infiltrating water at that point.
(vi) Measuring the differences in soil moisture and rise in water table.
(vii) By the general solution of the hydrologic equation for an area.
Infiltration due to rainfall, from irrigated and unirrigated lands, and seepage from reservoirs, lakes, ponds and canals constitute the ground water accretion.