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Interpreting Differences among Hydraulic Parameters for Different Soil Series by Functional Characterization

Department of Soil Science and Geology, Agricultural Univ., P.O. Box 37, 6700 AA Wageningen, the Netherlands

* Corresponding author ( booltink{at}rcl.wau.nl).

ABSTRACT

Using soil survey data for simulation modeling of water regimes and crop growth requires characteristic expressions for hydraulic characteristics of major soil horizons and soil series. It is unclear at present whether: (i) hydraulic functions based on parameter estimation techniques are adequate for simulations in macroporous soils, and (ii) to what extent different soil horizons and series can be pooled to reduce input data in simulation models. These questions were examined using two methods of expressing measured hydraulic conductivity data in computer simulations with previously validated crop models. Both hydraulic conductivity expressions used parameterized equations derived from one-step outflow data for the –5.0 to –60.0 kPa pressure range. For the 0 to –5.0 kPa range, one method used hydraulic conductivities estimated by extrapolating the parameterized functions, whereas the second method used actual crust test data to account for macropore flow. Differences between the two methods were functionally tested by simulating maize (Zea mays L.) yields and fluxes below the root zone in three Costa Rican soil series, using the SWACROP model. Yields were significantly higher when considering structural porosity (Method 2), while fluxes were significantly lower. Thus, extrapolation of one-step outflow data to conditions near saturation seems to be questionable. Using Method 2, maize yields and fluxes below the root zone were compared for three representative soil series to test whether they could be considered as one functional group of soils. Yield data indicated that two rather than one functional group should be distinguished. Flux data were similar for all three series, indicating that only one functional group was necessary.

Received for publication October 28, 1993.