Though it has nothing to do with parameter estimation, LUMPREM is a member of the PEST suite; it can be downloaded from the PEST web pages.
LUMPREM is a simple, lumped-parameter recharge model. It generates time series of groundwater recharge using daily rainfall and potential evaporation as inputs. It can also be used to simulate crop water demand and residual potential evaporation, i.e. the potential evaporation that is available to the groundwater system after the demands of the unsaturated zone have been met. As well as this, it can generate proxy time series of groundwater levels that can be applied to boundaries of a groundwater model.
This tutorial covers construction of LUMPREM model input files for different land-use and irrigation characteristics. It then discusses how to use the LR2SERIES utility to translate LUMPREM-generated time series to formats that the RECHARGE, EVT and GHB packages of MODFLOW 6 can read. Lastly, it demonstrates how to manipulate MODFLOW 6 time series files using the TS6PROC utility. LR2SERIES and TS6PROC are supplied with LUMPREM.