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Agri-Meteorological Data

Soil Moisture Deficits, Evaporation, Potential Evapotranspiration, Actual Evapotranspiration and Runoff :
  • Soil Moisture Deficits (SMDs) refers to 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 is surplus to field capacity and there is no capacity for infiltration i.e. any rain will run off. Saturation is reached when SMD= -10. Positive SMD is below field capacity and rain can infiltrate to the capacity of the SMD amount. Wilting Point is where the SMD reaches a value at which the plants cannot extract any water from the soil, grass has a wilting point of 120mm (approx.). The rate of water loss equals the potential rate for SMD's less than 30mm. Daily soil Moisture Deficits are available for our Synoptic Stations. Soil moisture deficits and surpluses are computed from the differences between rainfall and actual evapotranspiration. SMD surpluses are assumed to be removed by drainage and surface run-off and are not therefore carried forward from one period to the next. Where heavy rain occurs near the end of the fixed period the date of cut-off may be adjusted to avoid error due to insufficient run-off time.
    Evaporation measures 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 at our Current Evaporation Stations. 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 Evapotranspiration (PE) refers to the water flux under unlimited 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 dept. 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 Kinsealy, Co. Dublin, Johnstown Castle, Co. Wexford and Valentia Observatory, Co.Kerry. The Penman formula is used to calculate the daily Potential Evapotranspiration by using meteorological data such as air temperature, solar radiation, relative humidity and wind speed recorded at our Synoptic Stations. Actual Evapotranspiration is the water flux which actually occurs (soil limited). Estimates of Actual Evapotranspiration are derived from measured values of Potential Evapotranspiration and SMD's.
  • Runoff refers to the water leaving an area and flowing across the land surface to points of lower elevation. It consists of precipitation that neither evaporates, transpires nor penetrates the surface to become groundwater.

Formula used in calculating :

Potential Evapotranspiration
PE = 6.5(0.41 + 0.014T)G/(2502 - 2.38T)
Where: PE :potential evapotranspiration
T :mean temperature
G :global radiation

Actual Evaporation SMD < 30 => AE = PE
SMD > 30 => AE = PE * (smd_max - smd) /(smd_max - smd_thres)
Where: SMD_MAX :120mm
AE :actual evapotranspiration
PE :potential evapotranspiration
SMD :initial SMD

Soil Moisture Deficit new smd = initial smd + AE - rain + runoff
Where: AE :actual evaportranspiration
intial SMD :SMD from the previous day

Runoff SMD < 0 => Runoff = -SMD
maximum runoff : 3mm
If the SMD < -10mm(min)
Saturation excess of runoff = -SMD - 10mm
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