Development and evaluation of a lower-boundary-controlled weighing lysimeter: measuring evaporation under field conditions

Abstract

Accurate field measurement of evaporation requires weighing lysimeters that reproduce realistic hydraulic boundary conditions while operating reliably with minimal maintenance. This is particularly challenging in coastal reclaimed fields, where shallow and rapidly fluctuating groundwater and desalinization practices can drive the soil profile toward near saturation, limiting the performance of conventional equilibrium tension lysimeters that rely on vacuum regulation. We developed a Lower-Boundary-ControlledWeighing Lysimeter (LBC-WL) that regulates the lower boundary through bidirectional water exchange using a bidirectional tubing (peristaltic) pumping system and operates autonomously with a compact solar panel power supply. The system consists of a weighing inner cylinder whose base contains porous ceramic cups embedded in a silica sand layer and connected to an external water tank. A datalogger continuously compares the bottom pressure head inside the lysimeter (hb) with an external reference head measured outside (hout) and actuates the pump to maintain hb within a prescribed tolerance band around hout, with a short waiting time after each pumping action to improve stability. Laboratory single-step and multi-step tests demonstrated that the pumping system maintained hb close to prescribed targets over both relatively dry and near-saturated ranges while conserving mass balance between the lysimeter and the tank, consistent with the water retention behavior of the silica sand. Greenhouse tests confirmed continuous operation under solar panel power and showed that widening the tolerance band under near-saturated conditions reduced excessive pump cycling. Field deployment during desalinization in a coastal reclaimed area maintained lower boundary control (typically hb = hout±10 to ±15 cm, depending on wetness) and enabled evaporation to be quantified at 10-min resolution during the subsequent drying period. These results indicate that the LBC-WL provides a robust platform for studying evaporation and water balance processes in managed fields with shallow groundwater.