Passive wick fluxmeters: Design considerations and field applications
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 < 100 mm/yr. Passive-wick fluxmeters with 60-cm high control tubes and 60-cm long wicks were tested in soils ranging from non-vegetated semi-arid settings in the USA to grasslands in Germany and rain-fed crops in New Zealand and the South Pacific. Where side-by-side comparisons of drainage were made between passive-wick fluxmeters and conventional lysimeters in the USA and Germany, agreement was very good. In semi-arid settings, drainage was found to depend upon precipitation distribution, surface soil, topographic relief, and the type and amount of vegetation. In Washington State, USA, soil texture dominated all factors controlling drainage from test landfill covers. As expected, drainage was greatest (> 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.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 953783
- Report Number(s):
- PNNL-SA-60335; WRERAQ; TRN: US201004%%542
- Journal Information:
- Water Resources Research, 45:Article Number: W04420, Vol. 45; ISSN 0043-1397
- Country of Publication:
- United States
- Language:
- English
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