The Water Balance of an Agricultural Catchment. I. Estimation of Evaporation From Soil Water Records

Abstract

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 2 5 mm. Larger variability i s 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 asscssing the evaporation for a particular site.

The diurnal course of soil moisture as measured by various dielectric sensors: Effects of soil temperature and the implications for evaporation estimates

“…Furthermore, D can be found by employing a time-integrated form of Darcy's law (Klaij and Vachaud, 1992). Alternatively, the distinction between E and D can be made by determination of the zero-flux-plane, ZFP (McGowan and Williams, 1980), above which E occurs and below which drainage takes place.…”

Water balance and crop coefficient estimation of a citrus orchard in Uruguay

“…The water balance (WB) method has been widely used in citrus (Koo and Sites, 1955;Koo, 1961;Kalma and Stanhill, 1969;Hoffman et al, 1982;Smajstrla et al, 1982;Wiegand and Swanson, 1982;Sharples et al, 1985;Castel et al, 1987;Metochis, 1989;Castel andBuj, 1990, 1993;Morgan, 1992;Durán, 1993;Sepaskhah and Kashefipour, 1995;Fares and Alva, 1999;Grismer, 2000) and in other crops (McGowan and Williams, 1980;Vachaud et al, 1985;Moreno et al, 1988;Andreu et al, 1997).…”

“…With regard to the last parameter the results of McGowan and Williams (1980) with rain fed wheat (Triticum aestivum L.) should be considered. These authors found large variation in changes of stored soil water, measured in profile replicates.…”

Comparison of neutron scattering and DFM capacitance instruments in measuring soil water evaporation

“…This can hamper the NWM from sensing small changes in soil water content brought about by evaporation from the soil surface. Secondly, the NWM reading errors mostly occur in the top horizon of the soil profile where there is a possibility of radiation escaping through the surface and hence not giving a fully representative reading of the water content (McGowan and Williams, 1980). This failure of the NWM to observe daily change in water content, caused (Evett et al, 2002;Heng et al, 2002;Mwale et al, 2005;Mounzer et al, 2008;Vera et al, 2009).…”

“…The NWM has been used to measure soil water content at different points in time and, if all the other components of the soil water balance equation are measured, it is possible to calculate the loss of soil water through evaporation from the soil surface (McGowan and Williams, 1980;Bennie et al, 1994;1998). With the application of the same principle of monitoring change, some studies quantifying evaporation using capacitance probes have also been done (Verhoef et al, 2006;Mounzer et al, 2008).…”