Soil Infiltration PDF

By: ER Lapong ABE Department, MSU-GSC  Infiltration is the passage of water into the soil surface. In general, infiltration has a high initial rate that diminishes with time. It is a key to successful irrigation and water conservation in rainfed agriculture. It is also an...

By: ER Lapong ABE Department, MSU-GSC  Infiltration is the passage of water into the soil surface. In general, infiltration has a high initial rate that diminishes with time. It is a key to successful irrigation and water conservation in rainfed agriculture. It is also an important hydrologic process that must be considered in drainage and flood control work.  FACTORS AFFECTING INFILTRATION Infiltration may be considered as a three-stage sequence. (1)surface entry (2)filling up of soil profile storage potential, and (3)movement or transmission of water within the soil profile * The three phases are interactive in nature. (1) The fluid or water Specific fluid properties influencing infiltration ✓ Viscosity ✓ Turbidity ✓ Depth of water over the soil surface (2) The porous medium or soil Soil properties affecting infiltration ◆ Porosity ◆ Structure ◆ surface stability ◆ moisture holding capacity ◆ antecedent soil moisture ◆ mineralogy Lateritic soils are non-swelling and are usually characterized by high infiltration rates. Montmorillonitic clay soils crack upon drying and have high initial infiltration rates. They however, swell upon wetting and, hence, have very low permeabilities. (3) Vegetation and soil cultural practices - are the most significant factors affecting infiltration Vegetal cover protects the soil against raindrops energy and improves soil structure through organic matter production and root penetration. Tillage practices loosen up the upper soil layer thereby tremendously increasing the rate of surface entry of water and the porosity of the plowed layer.  MEASUREMENT OF INFILTRATION 1. Infiltrometers 2. Basin Method 3. Watershed Hydrograph Method 1. Infiltrometers double ring infiltrometers double ring infiltrometers 2. Basin Method The basin method is similar to flooding except that a large area is ponded. The rate of drop in the depth of ponded water is measured with time. This method is not practical and is very seldom used. Also, it is very difficult to measure the initial infiltration rates.  3. Watershed Hydrograph Method - characterizing infiltration from watersheds through the analysis of runoff and rainfall from watersheds’ drainage areas. The procedure is to subtract surface runoff rates, which involves hydrograph analyses to separate surface runoff from total streamflow as well as making corrections for evapotranspiration and surface and channel detention storages. Its main advantage is that the resulting infiltration data is representative of that of an entire catchment. This method is rather difficult and expensive to apply. Large amounts of information have to be collected and analyzed.  INFILTRATION EQUATION An infiltration equation may be expressed as: a. Instantaneous infiltration rate (f) b. Cumulative infiltration rate (i) 1. Gardner and Windsoe Equation (1921) i = C1t + C2 (1-eβt) Where: i = cumulative infiltration at time t C1, C2 and β = constants or parameters 2. Lewis-Kostiakov (1932, 1937) i = ctα f = αctα-1 Where: c and α are constants with the value of α in between zero and unity Lewis-Kostiakov equation implies that at t → ∞, f = 0. This is only possible if the soil is underlain by an impermeable layer. As such, the equation will only apply for soil underlain with impervious layers or for small t values in soils with pervious layers. log i = log c + α log t Hence, it will be very easy to test for the fit of infiltration test data into this equation. 3. Horton Equation (1940) Horton realized that infiltration rate is a decay type function whereby continuous infiltration and wetting of the soil will decrease infiltration due to decrease in soil profile storage potential, inwashing of fine soil particles into soil voids and swelling of soil colloids and closing of soil cracks. All these processes have the nature of an exhaustion phenomenon.  fo − fc  i = fc t +  (  1 − e − kt )  k  f = fc + (fo - fc)e-kt 4. Philip (1957) Philip analyzed infiltration as a general phenomenon of water movement in a porous medium. In particular, he described vertical infiltration in terms of the partial differential equation, The resulting equation is a rapidly converging series which can be truncated after the first two terms without much loss in accuracy. Thus, i = St1/2 + At S −1 / 2 f = t +A 2