Parameter Estimation from Stepped-Irrigation Tests on Instrumented Lysimeter Test Plots

Calibration of soil-atmosphere models to in situ measurements of suction, moisture content, and lysimeter outflow in response to atmospheric conditions can be time consuming. Calibration usually involves iterative and often arbitrary adjustment to estimated or laboratory-determined material properties until an acceptable match is achieved between predicted and measured responses to atmospheric conditions. Stepped irrigation and draindown tests on instrumented lysimeter test plots, however, can provide an improved basis for estimating bulk material properties for use in model development beyond that available from laboratory measurements or calibration to measurements during normal atmospheric responses. Irrigation tests provide, in part, information regarding the change in the volumetric moisture content and unsaturated hydraulic conductivity. The relationship can be directly measured during the test. This provides valuable information for use in calibrating a numerical model, specifically for capturing scale effects and constraining the relationship where standard models based on a proposed soil water characteristic curve are not valid. This paper describes a stepped-irrigation test conducted on an instrumented lysimeter in a mine waste rock pile. The irrigation portion of the test was conducted for seven days at two application rates. The draindown portion was monitored for 14 days following the test. The soil-atmosphere model based on the parameters from the irrigation test achieved a closer match to the observed conditions than the model calibrated to in situ measurements alone.