RETMCL: Incorporating maximum-likelihood estimation principles in the RETC soil hydraulic parameter estimation code

Abstract

RETC is a public domain computer code for estimating parameters of the water retention curve and hydraulic conductivity functions of unsaturated soils. RETC was developed at the US Salinity Laboratory and is now used world-wide with thousands of copies distributed. Evaluation of the final estimation results in the code has been improved to yield a new version, RETCML, based on the maximum-likelihood theory for the special case of weighted least-squares estimators. This paper first explains the theory of maximum-likelihood and introduces the principles of model adequacy and parameter uncertainty on a formal basis. Next, this paper presents a user guide for the code. RETCML is also free and has been programmed to be almost fully compatible with the original RETC input files, thus making it easy to re-analyze previous data. The output of RETCML includes a thorough evaluation of estimation results.