The Effect of Precipitation Pulses on Evaporation of Deeply Buried Phreatic Water in Extra‐Arid Areas

Abstract

Core Ideas Precipitation may increase deeply buried phreatic evaporation (PE). Soil‐water hysteresis leads to this phenomenon. This study reveals the mechanism of the effect of precipitation pulses on PE. Past research has shown that evaporation of deeply buried phreatic water occurs in extra‐arid areas where the soil surface is very dry. A dry surface layer can suppress the evaporation of capillary water, but it is not clear whether it suppresses deeply buried phreatic evaporation (PE). We deduced that the PE could be suppressed by a dry surface soil. Therefore, as the soil‐water content increases during a precipitation pulse, this can lead to an increase in PE. To investigate the effects of precipitation pulses on PE, a simulated 10‐mm precipitation recycling experiment using a shed–air‐conditioning condensation method was performed on Gobi Desert land at the top of the Mogao Grottoes in 2011 to 2015. The results show that after the 10 mm of precipitation was completely recycled, the PE increased by 28.9% compared with the control during the following 3 yr. In addition, the soil moisture increased in the 10‐ to 20‐cm layer for a long time. We hypothesize that this is a soil‐water hysteresis phenomenon, which is important for the observed differences in the evaporation rate, and it is apparent that the higher water content in the shallow layer is conducive to PE entering into the atmosphere. This preliminary study reveals the effect of precipitation pulses on deeply buried PE; this hysteresis phenomenon must be analyzed in further studies.

Analytical estimation show low depth‐independent water loss due to vapor flux from deep aquifers

“…To give a longer time scale interpretation, in 1000 years, we predict about 10 mm of vapor loss from the water table, which for an aquifer with 0.25 drainable porosity would lead to a decline of about 25 mm per millennium, or 25 m per million years, suggesting that unless the system had been under arid conditions for many million years, the vapor flux from the aquifer would likely not be the key factor in establishing the depth to groundwater, again, in sharp contrast to the findings of Li et al [2010, , and Li et al [2016]. We hope that these computations illustrate the utility of reasonable bounds on hydrological processes when considering either numerical or experimental results.…”

“…[], Li and Wang [], and Li et al . []. We hope that these computations illustrate the utility of reasonable bounds on hydrological processes when considering either numerical or experimental results.…”

“…It has been wondered recently how much evaporation due to vapor diffusion might occur from a water table that was on the order of 200 m below the surface of a desert in absence of net recharge [e.g., Ross , ; Walvoord et al ., ; Scanlon et al ., ; Li et al ., ]. It is somewhat disconcerting that such a basic aspect of many desert environments remains a matter of divergent predictions, with estimates that have yet to be reconciled spanning a range of 0.0001–5 mm/yr under otherwise consistent conditions [ Ross , ; Scanlon et al ., ; Li et al ., ]. Given the challenges of making experimental measurements [e.g., Li et al ., ; Li and Wang , ; Li et al ., ], and the lack of feasibility to generically verify numerical analyses of the problem [ Scanlon et al ., ; Walvoord et al ., ], it is useful to recognize that the calculation can be completed in a very general manner analytically.…”

“…It is somewhat disconcerting that such a basic aspect of many desert environments remains a matter of divergent predictions, with estimates that have yet to be reconciled spanning a range of 0.0001–5 mm/yr under otherwise consistent conditions [ Ross , ; Scanlon et al ., ; Li et al ., ]. Given the challenges of making experimental measurements [e.g., Li et al ., ; Li and Wang , ; Li et al ., ], and the lack of feasibility to generically verify numerical analyses of the problem [ Scanlon et al ., ; Walvoord et al ., ], it is useful to recognize that the calculation can be completed in a very general manner analytically. We seek here to bound the result by computing with accuracy to one significant figure (on the order of ±100%), so that we might reduce the range of values under consideration, while pointing out the dimensions and site features that control this flux.…”

Evaporation From Deep Aquifers in Arid Regions: Analytical Model for Combined Liquid and Vapor Water Fluxes

“…However, these upward water fluxes are extremely low, thereby posing great challenges for accurate quantification. Reported rates from few studies that attempted to measure evaporation in such deserts are erratic, varying by up to 5 orders of magnitude, between 10 −2 and 10 2 mm per year (Li et al, , , ; Li & Wang, ). On the other hand, modeling these rates poses complexity challenges and is computationally demanding, as it involves processes of water flow in liquid and vapor phases over large temporal and spatial scales, in heterogeneous media.…”