Agri-Meteorological Data
Soil Moisture Deficits, Evaporation, Potential Evapotranspiration, Actual Evapotranspiration and Runoff:
- Soil Moisture Deficit (SMD): this 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.
*Referrences
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. |