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Soil Moisture Deficits, Evaporation, Potential Evapotranspiration, Actual Evapotranspiration and Runoff:
- Soil Moisture Deficit (SMD) is the amount of rain needed to bring the soil moisture content back to field capacity.
- Field capacity (SMD=0) is the amount of water the soil can hold against gravity i.e. the maximum water a pot plant can be watered and not leak water. Negative SMD indicates a water surplus, which will be drained over time through either infiltration or overland flow or both.
- Saturation is reached when SMD= -10mm, i.e a water surplus of 10mm. Positive SMD is below field capacity and rain can infiltrate to the capacity of the SMD amount. In a saturated soil all of the available soil pores are full of water, but water will drain out of large pores under the force of gravity.
- Evaporation is the rate of water loss from a free water surface such as a reservoir, lake, pool, or saturated soil. A Class A Pan is used for measuring evaporation. This is a circular tank 1.21 m in diameter and 0.25m deep, partly filled with water and mounted on a frame to allow free circulation of air underneath. Additional water is required to maintain a set level.
- Evapotranspiration is the total water flux into the atmosphere, i.e. the sum of evaporation and transpiration (water flux through plant stomata).
- Potential (or Reference) Evapotranspiration (PE) is the water flux under non-limiting soil water conditions. A lysimeter is used to measure the rate of potential evapotranspiration from grass. It consists of four sunken tanks, each some 0.25m sq. in area and 0.75m in depth. The soil surface in each tank is at the same level as the surroundings. Grass cover is maintained on the tanks. Measured Potential Evapotranspiration is recorded at Johnstown Castle, Co. Wexford and Valentia Observatory, Co.Kerry. In Met Éireann the FAO Penman-Monteith formula is used to calculate the daily Potential (Reference) Evapotranspiration, using meteorological data recorded at our Synoptic Stations.
- Actual Evapotranspiration (AE) is the water flux which actually occurs. This is limited by the amount of moisture available in the soil. Estimates of Actual Evapotranspiration are derived from calculated values of Potential Evapotranspiration and current SMDs.
Soil Moisture Deficit Model
A hybrid SMD Model* has been developed which accounts for differences in drainage regimes between different soil types in Ireland. Three soil drainage classes, well drained, moderately drained and poorly drained, are defined as follows:
- Drainage is the amount of water lost from the topsoil through either percolation or overland flow and is dependent on the soil drainage capacity.
- Well Drained: Soil never saturates, remains at field capacity even on very wet days in winter. Minimum SMD=zero. When SMD > 0mm AE is less than PE, decreasing linearly to zero when SMD is at a theoretical Maximum of 110mm.
- Moderately Drained: May saturate on wet winter days, but return to Field Capacity on first dry day. Minimum SMD= minus10mm. When SMD >0 AE is less than PE, decreasing linearly to zero when SMD is at a theoretical Maximum of 110mm.
- Poorly Drained: Saturates on wet winter days, water surplus is drained at very slow rates, in the order of 0.5mm per day. Minimum SMD= minus 10mm. When SMD >10mm AE is less than PE, decreasing linearly to zero when SMD is at a theoretical Maximum of 110mm
Daily Soil Moisture Deficits and calculated PE and AE are available for the three different soil drainage classes for our Synoptic Stations. Soil moisture deficits and surpluses are computed from the differences between rainfall and actual evapotranspiration. Soil moisture surpluses are assumed to be removed by drainage and surface run-off over time.
Potential (Reference) Evapotranspiration Calculation
The potential evapotranspiration, ET0 is calculated according to the FAO Penman-Monteith Equation (Allen et al., 1998) for a reference grass crop at an assumed height of 0.12m :
where ET0 is the potential evapotranspiration (mm d-1),
Rn is the net radiation at the crop surface (MJ m-2 d-1),
G is the ground heat flux density (MJ m-2 d-1),
T is the air temperature at 2 m height (°C),
u2 is the wind speed at 2 m height (m s-1),
es and ea are the saturation vapour pressure and the actual vapour pressure, respectively (kPa),
Δ is the slope of the vapour pressure curve (kPa °C-1),
and γ is the psychrometric constant (kPa °C-1).
Soil Moisture Deficit Calculation
The Soil Moisture Deficit (SMD)is calculated as follows:
where SMDt and SMDt-1 are the SMDs on day t and day t-1 respectively (mm),
Rain is the daily precipitation (mm d-1),
ETa the daily actual evapotranspiration (mm d-1),
Drain the amount of water drained daily by percolation and/or overland flow (mm d-1)
Actual Evaportranspiration
For each soil drainage class a critical Soil Moisture Deficit, SMDc, is defined. When the current SMD is less than this critical value then moisture is not limiting respiration and Actual Evapotranspiration equals to Potential Evapotranspiration:
When SMD <= SMDc
When the current SMD is greater than this critical value, moisture available is no longer unlimiting; as a result, Actual Evapotranspiration is less than Potential Evapotranspiration. In this case it is assumed that AE deceases linearly to zero as the SMD approaches a theoretical maximum value, SMDmax.
When SMD > SMDc
The value of SMDc for well and moderately drained soils is zero, and 10mm for poorly drained soils. The value of SMDmax is 110mm for all three soil types.
Drainage
It is assumed that drainage by means of percolation or overland flow only occurs when soil moisture exceeds field capacity (SMD < zero).
- Well Drained soils: These remain at field capacity even on very wet days and are never saturated, all water in excess of field capacity is drained immediately.
- Moderately Drained Soils: These carry water surpluses on wet days but return to field capacity on the first subsequent dry day. This corresponds to a maximum drainage rate in excess of 10mm/day.
- Poorly Drained Soil: These can carry surplus water for a number of days, water drains at the maximum rate of 0.5mm/day when SMD is -10, decreasing linearly to zero when SMD > zero
Effective Drainage is the total drainage amount, i.e. through percolation and overland flow.
*References
- Allen, R.G., Pereira, L.S., Raes, D. and Smith, M. 1998. Crop evapotranspiration. Guidelines for computing crop water requirements. FAO irrigation and drainage paper 56, 227 pages.
- R.P.O.Schulte, J.Diamond, K.Finkele, N.M.Holden and A.J.Brereton 2005. Predicting the Soil Moisture Conditions of Irish Grasslands. Irish Journal of Agricultural Research 44: 95-110.