Groundwater Recharge Calculations as Affected by Temporal Averaging of Meteorological Data

Groundwater Recharge Calculations as Affected by Temporal Averaging of Meteorological Data

presented by Martinus van Genuchten

Simulations of water flow rates into and through the near-surface vadose zone can provide useful estimates of deep drainage and groundwater recharge. Past studies have shown that results can be very sensitive to the temporal resolution of meteorological data used in the simulations. In this study we used the HYDRUS-1D software to quantify the effects of using weather-related data of different resolutions in time (hourly, daily, weekly, monthly and yearly averaged precipitation and potential evaporation rates) on predictions of groundwater recharge at three sites in Brazil having different climatological conditions: a tropical savanna (the Cerrado), a humid subtropical area (the temperate south of Brazil), and a very wet tropical area (Amazonia). Our analysis was limited to evaporation processes and hence did not consider any land use effects by ignoring root water uptake. Temporal averaging was found to lead to very different predictions of recharge, with the yearly averaged data always producing by far the lowest recharge rates. Results were especially sensitive to averaging when applied to the Cerrado site for which the daily averaged weather data produced up to 9 times the recharge rates obtained with yearly averaged data. The effects at the Cerrado site are due to the very uneven rainfall distribution over the year involving distinct wet and dry seasons. Long-term averaging can lead to especially poor recharge estimates for coarse-textured soils since averaging limits deep penetration of moisture in response to precipitation events. Infiltrated water will then remain in the upper part of the vadose zone and become subject to possible upward flow and evaporation. Accounting for short-term high-frequency precipitation events is also important when estimating surface runoff.