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GRAS (Climate) Climate National level National Soil Water Balance
National Soil Water Balance

In this section the results of the Soil Water Balance model for the soils of Uruguay are presented. This model estimates the available water content of a region taking into account the type of soil, the effective precipitation, the atmospheric potential demand of water and the vegetation transpiration.

In the following table are the outputs of the Soil Water Balance model since September 2000:

Outputs of the Soil Water Balance

This model uses the following as input variables:

·         PRE: Effective Precipitation is calculated substracting from the precipitation registered in 84 meteorological centres (79 of the DNM and 5 of INIA, see map), a superficial draining estimated by considering the preceding rainfalls (5 previous days).

·         ETP: Potential Evapotranspiration or pasture soil demand of water is calculated considering a physical model that estimates the potential evapotranspiration based on daily values of: Temperature, Air Humidity, Wind Speed and Solar Radiation. The method used is the so called Pemnan-Monteith.

·         CR Soils Water: Soil capacity to withhold water for each of the units of soil defined in the Uruguayan Soil Recognising Chart scale 1:1.000.000 (Renewable Natural Resource National Office, Land and Water Division, Ministry of Livestock, Agriculture and Fishery). The type of soil determines the maximum capacity for the soil to withhold water at the roots activity zone.

For each day the storage change is calculated through the application of a logarithmic model that considers the soil water withholding (adapted from Thornthwaite C.W. and Mather J.R. “Instructions and tables for computing potential evapotraspiration and the water balance”. Publ. in Climatology, 10:181-311, 1957).

The daily transpiration value corresponds to the storage change calculated in the previous step.

Output Model Variables:

·         IBH: Hydric Welfare Index that results from the percentage of transpiration in relation to the daily potential demand (ETR/ETP). IBH values near 1.0 show that the vegetation has transpiration values near to the potential demand. On the contrary, IBH values near 0.0 show that the vegetation has transpiration values much lower than the potential demand, showing that from a climatologic point of view the vegetation is under hydric stress.

·         ETR: Real evapotranspiration.

·         ANR: Non retained water, is the sum of the Superficial draining and Water excess in soil (Water that exceeds the water content of soil at field capacity).

·         ADI: content of available water in soil.

·         PAD: percentage of available water, as ADI/CC*100 (CC: field capacity).

Note: the IRRIGATION WATER NEED as % of CC is equal to (100-PAD).


The methodology considers that the vegetation of covering is:

·         A pasture (grassy type),

·         It intercepts 100% of incident radiation, and

·         Is under active growth.