EFFECT OF SOIL HEAT FLUX ON THE WATER BALANCE OF A SMALL CATCHMENT / L'effet du flux en chaleur du sol sur le bilan hydrologique d'un petit bassin versant

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Abstract
A study of the water balance of the upper River Ray basin reveals that calculated soil moisture storage exceeds measured storage in winter and falls well below it in summer. The systematic seasonal error is of the order ±50 mm. Several reasons have been proposed which may account for some local water balance anomalies. However, the reported discrepancies are characteristic of a much broader area, and are attributed by Edwards and Rodda (1970) partly to the omission of a soil heat storage term (G) from the Penman equation. The present analysis tests this hypothesis. Soil heat flux is measured directly at the Grendon Underwood climatological station and is found to be small in relation to other water balance components. Inclusion of the term in the Penman equation is not enough in itself to account for the published seasonal moisture anomalies. With a change of neutron probe from that used in the earlier study, and inclusion of G in the calculations, the 50 mm imbalance can be reduced to +34 mm and −18 mm for the summer and winter half years respectively.