Evaporation and Infiltration

Why it matters

Evaporation and infiltration are crucial components of the hydrological cycle, influencing water availability for agriculture, industry, and domestic use. Understanding these processes helps in effective water resources management, especially in regions with variable rainfall patterns.

Key ideas

• Evaporation: The process by which water is converted from liquid to vapor and transferred to the atmosphere. It is influenced by factors such as temperature, humidity, wind speed, and solar radiation.
• Infiltration: The process by which water on the ground surface enters the soil. It is affected by soil type, vegetation cover, land use, and soil moisture content.
• Factors affecting evaporation:
  • Temperature: Higher temperatures increase evaporation rates.
  • Humidity: Lower humidity levels lead to higher evaporation.
  • Wind speed: Increased wind speed enhances evaporation by removing saturated air.
  • Solar radiation: More solar energy increases evaporation.
• Factors affecting infiltration:
  • Soil texture: Sandy soils have higher infiltration rates than clayey soils.
  • Vegetation: Dense vegetation can enhance infiltration by reducing runoff.
  • Land use: Urbanization can decrease infiltration due to impervious surfaces.

Formulas

• Horton infiltration-capacity model: f(t) = f_c + (f_0 − f_c)e^(−kt). Use f in mm/h, t in hours and k in h⁻¹. Parameters are calibrated for the soil/conditions. Actual infiltration is limited by available surface water as well as capacity.
• Pan method for reference evapotranspiration: ET_0 = K_p E_pan, with both depths over the same period. K_p depends on pan type, siting and weather; reference ET is not identical to open-water evaporation.

Worked examples

For f_0 = 40 mm/h, f_c = 10 mm/h, k = 1 h⁻¹ and t = 2 h of continuous ponding: f(2) = 10 + 30e^(−2) = 14.06 mm/h. This is an instantaneous capacity, not cumulative infiltration. Under continuous ponding, cumulative infiltration is F(t) = f_c t + (f_0 − f_c)(1 − e^(−kt))/k; here F(2) = 45.94 mm.

For measured pan evaporation 6 mm/day and a supplied appropriate coefficient K_p = 0.70, ET_0 = 0.70 × 6 = 4.2 mm/day.

Answers: 14.06 mm/h infiltration capacity at two hours, 45.94 mm cumulative infiltration under the stated ponding condition, and 4.2 mm/day reference ET for the separate pan example.

Common mistakes

• Confusing units, especially when converting between mm, cm, and m.
• Ignoring the impact of local climate conditions on evaporation and infiltration.
• Misapplying formulas by not considering all influencing factors.

For GATE CE

Questions often involve calculating evaporation or infiltration rates using given data. Practice problems involving different climatic and soil conditions to understand the variability in these processes.

Quick check

1. What factors increase evaporation rates?
2. How does soil texture affect infiltration?
3. Why is understanding infiltration important in urban planning?

Answers: 1. Temperature, wind speed, solar radiation, low humidity. 2. Sandy soils increase infiltration rates. 3. To manage stormwater and reduce flooding risks.

References

EPA SWMM hydrology manual; FAO pan method.