Seasonal groundwater storage anomaly and vadose zone soil moisture as indicators of precipitation recharge in the piedmont region of Taihang Mountain, North China Plain

Precipitation has been recognized as one of the most important sources of groundwater recharge in the piedmont region of Taihang Mountain, North China Plain. However, a number of recent studies suggest the possibility of precipitation recharge in this region. This is because evapotranspiration from the intensive irrigated farming is several orders of magnitude of the annual precipitation. To reappraise potential precipitation recharge in the region, observed changes in seasonal groundwater storage and soil moisture of three soil core samples (each exceeding 10 m deep) from a large soil-probe machine were analysed. Our analysis showed that groundwater storage in the winter/dry season (in the piedmont region) is influenced mainly by lateral recharge and the amount of precipitation in the preceding year. In the summer/rainy seasons, groundwater storage differs with precipitation year. With the exception of extraordinary rainstorm years (e.g. 1996 after the storm with 30 year return period), precipitation recharge is not apparent. Soil field capacity analysis also revealed a possibility of precipitation recharge, with a wide margin between actual soil moisture and soil moisture content at field capacity. Precipitation recharge is only feasible when a single or successive precipitation is in excess of the minima (170 mm), a very rare condition in the piedmont region, where precipitation is easily depleted by the water-scavenging field crops, which cover two-thirds of the total land area.