Comparing different methods for estimating the soil moisture supply capacity of a soil series subjected to different types of management

The capacity of soils to supply water to growing plants was expressed in a static and dynamic manner by calculating 'available water' (AW) and the 'so il moisture supply capacity' (MSC), respectively. Four methods of increasin g complexity were compared for the AW and two for the MSG, using measured m oisture retention and hydraulic conductivity curves and data derived from c lass- and continuous pedotransferfunctions. Calculations were made for a pr ime agricultural soil in the Netherlands, a fine, mixed, mesic Typic Fluvaq uent (genoform). Three phenoforms, defined by long-duration management, wer e distinguished: BIO (biodynamic); CONV (conventional, high-tech) and PERM (permanent grassland). Values for AW for a given treatment were, in most ca ses, significantly different when using measured data or pedotransferfuncti ons. AW can be used to rank different soils but does not reflect the amount of water the crop can take up. Simulation models are needed to estimate MS C which cannot directly be measured. Simple empirical models, using AW for the rootzone in a 'tipping-bucket' approach, did not produce realistic valu es because upward flow from the relatively shallow water table could not be distinguished. The more complex mechanistic WAVE model, also including hyd raulic conductivity data, but still operating under the implicit and incorr ect assumption of soil homogeneity and isotropy, produced soil water conten ts that were too high. Realistic values were only obtained when considering bypass flow, internal catchment and accessibility of peds for rooting, usi ng a modified WAVE model incorporating hydraulic data derived from continuo us pedotransferfunctions. Values for AW and MSC were, in most cases, signif icantly different for the three phenoforms, illustrating the need to distin guish phenoforms. rather than only genoforms, when reporting basic physical data for soil series. (C) 1999 Elsevier Science B.V. All rights reserved.