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Use of Soil Survey Data in a Model for Simulating Regional Soil Moisture Regimes¹

ABSTRACT

Depth to water table and evapotranspiration rates were simulated with the computer program GELGAM for an area of 6 by 6 km, which was described by square nodal areas of 25 ha each in a grid with a mesh width of 500 m. Soils which occupied the largest part within a nodal area (as determined by analyzing a soil map) were considered representative for the nodal area and were characterized by basic data in terms of rooting depth, moisture retention curves and hydraulic conductivity curves. Four simulation runs were made with GELGAM, using increasingly detailed and costly basic data obtained by estimates and measurements in four phases as follows: (i) identical estimates for all nodal areas; (ii) estimates based on an existing soil map; (iii) estimates from one on-site boring per nodal area, and (iv) measurements of physical data.

Estimates of physical data were based on extrapolation of data obtained for similar soil horizons elsewhere, using empirical relationships with soil texture, organic matter content, and bulk density. Such estimates, when based on descriptions of mapping units in the soil survey report, were good for surface horizons but poor for the deeper subsoil beyond the solum in the region of the fluctuating water table. Simulated depth to water table agreed well with measured depths in all phases, but only when using measured physical data for the deeper subsoil. Simulated evapotranspiration using phase 1 data produced poor results for individual nodal areas when compared with phase 4 data (reference level), while phase 2 data produced good results if measured physical data for the deeper subsoil were included. Physical variability of nodal areas was better characterized by the soil survey data than by results from a single boring (phase 3).

Use of existing soil survey data, in combination with selected on-site investigations, could reduce the cost of data gathering in this study by an estimated 50% as compared with the common procedure of making systematic borings in each grid point. The degree of detail of interpretation of simulation results for nodal areas should be limited by the field variability observed.

¹ Contribution from the Netherlands Soil Survey Institute, Box 98, 6700 AB, Wageningen. This work was supported by a Grant from the Provincial Water Board of Gelderland, the Netherlands.

² Soil Scientists, Nederlands Soil Survey Institute.

³ Agrohydrologist, Int. Inst. for Hydraulic and Environmental Engineering. Delft.

⁴ Hydrologist, National Hydrolog. Inst. (Rijkswaterstaat) The Hague.

⁵ Hydrologist, Provincial Water Board of Gelderland, Arnhem.

Received for publication November 19, 1979. Accepted for publication March 10, 1980.