Evaluating a Drainage Model Using Soil Hydraulic Parameters Derived by Various Methods

Abstract

Soil hydraulic parameters are often indispensable input in hydrological modeling. Therequired input parameters can be obtained by measuring soil texture, bulk density, organic mattercontent, soil water retention and hydraulic conductivity. To minimize soil measurements, informationis needed on how well hydrologic models perform with varying levels of soil hydraulic parameters.The objective of this study is to determine which level of soil information would be sufficient to usewith DRAINMOD in predicting subsurface drainage volumes. Three groups of parameters wereobtained by various methods: 1) determining the soil texture and bulk density (BD) data from the SoilSurvey Database, then inputting them into a pedotransfer function model (ROSETTA) to determinesoil hydraulic parameters (denoted as SP_1); 2) analyzing the soil texture and organic matter(OM)content in laboratory and deriving the BD, field capacity (33kPa) and wilting point (1500kPa) fromliterature, then inputting them into ROSETTA to determine soil hydraulic parameters (SP_2); and 3)calibrated soil hydraulic parameters based on initial inputs from the Soil Survey Database plusROSETTA (SP_3). Parameters obtained from these three methods were used with DRAINMOD under the same weather, crop and soil conditions for 14 consecutive years at the subsurfacedrainage plots located in Pocahontas County, IA. Predicted subsurface drainage based on thosethree levels of soil hydraulic parameter inputs were compared to the observed ones through fourstatistical measures: Root Mean Square Error (RMSE), Co-efficient of Mass Residual (CRM), Indexof Agreement (IoA) and Model Efficiency (EF). The statistical results indicated that output from SP_3had the best fit with respect to observed values during the calibration period (1990-1993) and thatfrom SP_2 has the best fit when considering all 14 years. However, all methods provided reliableestimates of subsurface drainage. ROSETTA in combination with Soil Survey offers a quick and easyway to derive the soil hydraulic parameters, which were found reliable for DRAINMOD simulations topredict long-term subsurface drainage volumes for the site studied.