Infiltration, Storage and Evapotranspiration

A rain event has several destinations

After rain, one patch stays wet while another dries quickly. A rain gauge describes water arriving above a surface; it does not tell us how much enters the soil, remains near roots or becomes available to a crop. Following the water explains the difference.

Choose an area, a soil depth and a time interval before making an account. Here the root zone is a fixed layer occupied by active roots. Water can enter through its upper surface, move upward from deeper soil or move sideways. It can leave through plant uptake, evaporation, downward drainage or lateral flow. Changing the chosen depth changes what counts as stored water.

Infiltration is entry, not permanent storage

Infiltration means water crossing the soil surface. The rate is often expressed as millimetres of water depth per hour. It varies with surface condition, existing wetness, pore connections and soil structure. Texture matters, but a texture name alone cannot predict a field rate. Crusting or compaction can restrict entry; connected pores can provide faster routes.

Water supplied faster than it enters may pond or run off. Water that infiltrates can still move below the selected roots. This deeper movement is percolation; crossing the lower root-zone boundary is drainage in this account. A wet surface therefore neither proves uniform wetting at depth nor guarantees that the whole application remains accessible to roots.

Stored water and accessible water differ

Soil retains water within pores and on surfaces. As soil dries, some remaining water is held too strongly, or moves too slowly toward roots, to meet plant demand. Roots also differ in depth, distribution and activity. Two layers containing equal water amounts can therefore provide different opportunities for uptake.

Field capacity describes a reference water content after relatively rapid drainage has slowed. It is not a universal waiting time or an exact boundary between healthy and unhealthy soil. Likewise, permanent wilting point is a reference used in estimating plant-available water, not a statement that all soil water has disappeared. Rooting depth, salinity and aeration help determine how these concepts apply.

Evapotranspiration joins two processes

Evaporation transfers liquid water from exposed wet surfaces into the air. Transpiration transfers water through a living plant and out mainly through leaf stomata. Their combined amount is evapotranspiration, abbreviated ET. Sunlight, air conditions, wind, canopy development and water supply influence it. ET is a water flux, not an amount of crop growth.

A weather-based reference ET describes a defined reference surface. Actual crop ET also depends on crop development and growing conditions. A stressed or waterlogged crop may use less water because growth or uptake is restricted; low ET does not automatically show efficient production. A water balance also needs to distinguish rain intercepted and evaporated above the soil from rain actually reaching the soil surface.

Keep inputs, outputs and storage together

For a simplified fixed root zone, write final storage = initial storage + rain + irrigation − runoff − ET − drainage. Here rain and irrigation are amounts reaching the chosen soil surface. Assume no capillary rise, lateral flow or material change in plant water storage. All entries refer to the same area and interval; runoff is subtracted before counting infiltrated water.

In an illustrative interval, initial soil-water storage is 40 mm. Rain adds 18 mm and irrigation 12 mm. Runoff removes 4 mm, ET 16 mm and drainage 4 mm. Thus 40 + 18 + 12 − 4 − 16 − 4 = 46 mm remains. The storage gain is 6 mm, not 30 mm. These supplied values demonstrate accounting; they are not a recommended moisture content or irrigation schedule.

Depth needs an area and a measurement method

A water depth represents volume spread over a named area. One millimetre over one square metre is one litre; one millimetre over one hectare is ten cubic metres. A depth per day is a rate, while a depth accumulated during a week is an amount. Keep those units separate when comparing a rainfall event with daily ET.

Measurements should cover the relevant soil layers and variable patches, with sensor calibration and timing recorded. Unmeasured drainage or capillary supply can make an apparent balance residual misleading. A closed numerical account is useful, but matching totals alone does not prove that every measured term is correct. Use it to identify missing flows and improve observations.

Worked examples

Check your understanding

1. Does all water that infiltrates stay within the selected root zone?

   Show answer and reasoning

   No.

   Some can cross the lower boundary by drainage or move laterally; infiltration describes entry rather than its final destination.

2. A dry layer still contains water. Does that prove uptake can meet crop demand?

   Show answer and reasoning

   No.

   Retention, movement toward roots, rooting and salinity affect access to the remaining water.

3. Can a low observed ET alone establish good water productivity?

   Show answer and reasoning

   No.

   It may reflect restricted growth or water uptake. Compare a defined useful output with a defined water denominator.

Key ideas

• Name the root-zone boundary, area and interval before adding water flows.
• Infiltration, stored water and accessible water answer different questions.
• ET combines evaporation and transpiration; actual crop conditions matter.
• Account for runoff, drainage and storage change instead of assigning all supply to the crop.