Modeling and analyzing water flow in a thick unsaturated zone during precipitation and infiltration

Abstract: With more and more attention being received in water flow through the widely distributed thick unsaturated zone in arid and semiarid region, the influence of thick unsaturated zone on the groundwater recharge is of important study. Quantification of groundwater recharge from precipitation under thick unsaturated zone condition can provide theoretic basis for analyzing the hydrological cycle with human disturbance, and reasonably establish water resources development and utilization. Based on the measured soil properties of a 9.6m-deep vadose zone core drilled by GEOPROBE from Caiyu town located in Xinfeng river basin in Daxing district in Beijing, this study focuses on simulation of one-dimensional water transport under 55-year long precipitation time series conditions, analysis of the spatio-temporal characteristics of the water flux and its response to the precipitation and soil texture, and discussion of the feasibility of estimating deep drainage from the shallow water flux. We aim to provide a simple and practical method for evaluation of the contribution of precipitation to groundwater recharge in bare soils. To achieve this, the mechanistic numerical model HYDRUS-1D based on the Richards' equation is used. First, annual precipitation and simulated evaporation, and deep drainage are calculated for the period 1951-2005, with an average annual precipitation of 597.59 mm, an average annual evaporation of 964.38 mm, and an average annual deep drainage of 131.03 mm. Variability in precipitation, ranging from 261.8mm to 1406 mm (Cv=35.72%), was amplified in deep drainage, ranging from 17.54 mm to 699.36 mm (Cv =88.03%), and was reduced in evaporation (Cv=4.58%). Then the long-term monthly average water flux of the soil profile is calculated for the period 1951-2005. The average monthly infiltration rate in the top layer of soil exceeds average monthly evaporation between May and September. Subsequently, long-term (1951-2005) monthly average rainfall, evaporation, and deep drainage for an extreme high (a), high (b), medium (c), and low (d) precipitation region are depicted. The average deep drainage is 103.55 mm in the medium precipitation region (575.76 mm/year average rainfall), with the coefficient of recharge from precipitation of 0.1799, which is lower than the 55-year average coefficient of recharge from precipitation of 0.2138. In general, the highest precipitation months are July and August, and average monthly precipitation exceeds average monthly evaporation during these months except for the low precipitation region. The highest evaporation months are from May to July due to high solar radiation and temperatures in these months. Finally, long-term average soil moisture fluxes in the whole soil profile corresponding to four different time periods (1996-2005, 1981-2005, 1966-2005, 1951-2005 respectively) are calculated, which reveals that the average infiltration rate on soil profile below 2 m keep almost unchanged with the variation of depth between 1951 and 2005.