Towards areal estimation of soil water content from point measurements: time and space stability of mean response

Areal estimates of soil moisture over large areas are required for the ground truthing of remotely sensed measurements and for establishing catchment-wide antecedent conditions for runoff simulations. There is a mismatch in scale between field (point) measurements and the areal estimates from both remote sensing and simulation modelling, so attention must be focused on developing sampling strategies that are able to determine accurate areal estimates of soil moisture using present (essentially point) techniques. In this paper, we use the concepts of time stability, applied to catchments with significant relief, to investigate the existence of certain parts of the landscape which consistently exhibit mean behaviour irrespective of the overall wetness. We denote these as catchment average soil moisture monitoring (CASMM) sites. Four data sets from three catchments (Tarrawarra, R5-Chickasha and Lockyersleigh) are examined. The catchments range in size from 10.5 ha to 27 km(2). Soil moisture measurements are made using time domain reflectrometry (TDR) or neutron moisture meters (NMMs), over depths from 30 to 120 cm. Time-stable locations representing mean areal moisture content are found in each catchment, i.e. CASMM sites exist. Although this analysis is preliminary, it points towards the possibility of a methodology for determining a sampling regime that could provide reliable estimates of areal mean soil moisture in complex terrain from a limited number of sample locations. (C) 1998 Elsevier Science B.V. All rights reserved.