THE WATER BALANCE OF AN AGRICULTURAL CATCHMENT. I. ESTIMATION OF EVAPORATION FROM SOIL WATER RECORDS

Errors of measurement of soil water content with the neutron probe can be made small enough for most water balance studies; and apart from the possible damage resulting from access tube implacement, they can be readily specified. A more important problem is estimating how much water is lost from soil by drainage. A simple, graphical method is described to distinguish between drainage and evaporation from soil, based upon identification of the maximum depth at which measurable quantities of water are extracted by roots. As a result of errors in soil water measurements and uncertainties in the distinction between drainage and evaporation, estimates of soil water deficit and hence evaporation for a single profile are likely to have a precision of about ±5 mm. Larger variability is commonly found in soil water storage changes measured in replicate profiles partly because of uneven wetting of the soil during rain and partly because of non-uniform losses by drainage and root extraction. This variability requires careful attention to replication and location of access tubes when assessing the evaporation for a particular site.