PNNL

Abstract

Optimization of water use in agriculture and quantification of leachate losses from landfills and watersheds require reliable estimates of vadose-zone water fluxes. Current technology is limited primarily to lysimeters, which directly measure water flux but may in some way disrupt flow, causing errors in the measured drainage. We report on design considerations and field tests of passive-wick lysimeters (fluxmeters) that use a control tube to minimize convergent or divergent flow. Design calculations with a quasi-three-dimensional model illustrate how convergence and divergence can be minimized for a range of soil and climatic conditions under steady-state and transient fluxes using control tubes of varying heights. There exists a critical recharge rate for a given wick length, where the recharge is 100% regardless of height of the control tube. Otherwise, convergent or divergent flow will occur, especially when the control tube height is small. While divergence is eliminated in coarse soils using control tubes, it is reduced but not eliminated in finer soils, particularly for flux rates 60% annual precipitation) from gravel surfaces and least (no drainage) from silt loam soils. In Oregon and New Mexico, USA, and in New Zealand, drainage showed substantial spatial variability. The New Mexico tests were located in semiarid canyon bottom terraces, with flash flood prone locations having extremely high drainage/precipitation ratios. In the wettest environments, drainage was found to be closely linked to rate and duration of precipitation events.