6.- Infiltration Dr. Luis E. Lesser All Tables and Figures (except where noted) were kindly provided by Pearson, from the textbook by David A. Chin, 2013. Water –Resources Engineering, 3rd edition. Infiltration • Infiltration or percolation is the processes by which water seeps into the ground through the soil surface (downward movement of water through soil ) • The rate of infiltration is directly influenced by the physical characteristics of the soil, soil cover, moisture content of the soil, soil temperature, precipitation rate (rainfall intensity) • Infiltration capacity is determined by soil cover and subsurface soil characteristics • When precipitation rate > infiltration capacity, surface runoff will occur For bare soil: High infiltration rate Low infiltration rate > 25 mm/hr < 2.5 mm/hr • Higher infiltration rate for soils covered with grass or other vegetation. • These may increase infiltration rates by a factor between 3 and 7.5 Infiltration • From the parameters involved in a meteorological water balance, infiltration is the most difficult to estimate and that carries the highest uncertainty. • For this reason, infiltration is usually determined from the meteorological water balance, as the unknown 푃 = 퐸푉푇 + 퐼푛푓 + 푅표 P = Precipitation EVT = Evapotranspiration Inf = Infiltration Ro = Runoff Infiltration • General rules of thumb for basin-scale infiltration estimates: o In average, 10% of precipitation infiltrates into the subsurface o In average, 30% of water used for agricultural irrigation infiltrates into the subsurface o There is also infiltration from public water distribution systems In Querétaro, leaks (or spills) amount possibly to 40% – 50% of supply The most efficient countries in terms of spills are (Fishman, 2011): U.S. – 16% loss Great Britain – 19% loss France – 26% loss • Local-scale infiltration is usually determined for agricultural purposes Soil Classification by size (texture) Soil Classification by size (texture) From: Davis, 1992 Soil Classification by size (texture) Mostly for agricultural applications FIGURE 9.18 USDA soil texture triangle. Bernoulli Equation 1 퐸 = 푃푉표푙 + 푚푧 + 푚푣2 2 Total Elastic Potential Kinetic Energy Energy Energy Energy (pressure) Divide by mg 퐸 푃 푉2 = + 푧 + 푚 휌 2 Tends to zero Potentials in the Subsurface in porous media 퐸 푃 푉2 = + 푧 + 푚 휌 2 h ψ z Hydraulic Pontential Pressure Pontential Elevation Pontential Hydraulic Head Pressure Head Elevation Head Hence: ℎ = ψ + 푧 In porous media, water moves from high hydraulic head (h) to low hydraulic head Potentials in the Subsurface 70 60 70 50 60 40 (z) (cm)(z) 50 30 (cm) 40 Elevation 20 30 10 Elevation 20 0 10 A B 0 0 10 20 30 40 50 60 70 h (cm) Potential (cm) Ψ (cm) Z (cm) Retention moisture curve or wetting curve θr = θ0 =residual moisture Suction (-ψ) θs = total saturation of porous media with water θ = n = porosity Hysterisis: dependence on its history Drainage (water out) – air displaces water in porous media Imbibition (water in) – water displaces air in porous media Lysimeter: measures negative pressures in porous media FIGURE 9.19 Typical moisture retention curve Infiltration process Minimum Infiltration rate is equal to the (vertical) saturated hydraulic conductivity of the soil FIGURE 9.20 Infiltration process. Infiltration process – Infiltration experiments 05_Derrame_gasolina.mov (3 min) Infiltration process – Estimation methods • Meteorological water balance • Rules of thumb basin-scale • Lysimeters • Infiltration experiments • Models: Horton Equation (1939;1940) Green-Ampt (1911, 1954) NRCS curve number method small-scale Philip Holton Infiltration process – Infiltration experiments https://www.youtube.com/watch?v=hWxLbwLf1WA&list=PLIh99V5LBWLo FhqnlRzjGH_D88-lK44tR&index=6 (1.5 min) Infiltration process - Horton Horton Equation (1939;1940) −풌풕 풇풑 = 풇풄 + 풇ퟎ − 풇풄 풆 풇풑 =Potential infiltration rate or infiltration capacity (in function of time) 풇풄 = Minimum infiltration rate 풇ퟎ = Initial infiltration rate (“real” maximum) 풌 = Decay constant 풕 = time Horton 풇풑 =Potential infiltration rate or infiltration capacity (in function of time) 풇풄 = Minimum infiltration rate 풇ퟎ = Initial infiltration rate (“real” maximum) 풌 = Decay constant 풕 = time FIGURE 9.21 Horton infiltration model. Horton Horton Equation (1939;1940) −풌풕 풇풑 = 풇풄 + 풇ퟎ − 풇풄 풆 푓 • Typically 0 = 5 푓푐 • Normally used for agronomy purposes – ok for small scales • Not practical for basin-scales (unrealistic) • It does not take into account spatial variability of hydraulic conductivity Horton Horton Horton Example 9.15 A catchment soil has the following Horton infiltration parameters: 풇ퟎ =100 mm/h 풇풄 = 20 mm/h 풌 = 2 min-1 a) What rainfall rate would result in ponding from the beggining of the storm? b) If the rainfall rate is maintained for 40 min, describe the infiltration as a function of time during the storm c) What is the infiltration rate 1, 2, 3 and 4 min after starting the storm? .