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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
SMD_THRES :30mm
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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