Using tracer tests to estimate vertical recharge and evaluate influencing factors for irrigated agricultural systems

Accurate estimation of groundwater recharge is critical for (semi) arid regions, especially in places like the North China Plain (NCP) where recharge from irrigation and intermittent precipitation events comprises the largest portion of recharge. Tracer tests were used to estimate potential recharge beneath agricultural systems irrigated by groundwater, and to help delineate factors that influence recharge. A bromide solution was applied below root regions to trace infiltration in the vadose zone beneath irrigated agricultural fields and non-irrigated woodlands at both piedmont plain (Shijiazhuang) and alluvial and lacustrine plain areas (Hengshui) in the NCP. The tracer tests lasted for more than 2 years and were conducted at 37 subsites grouped in sets of two to four at 12 regionally distributed sites. For the piedmont plain sites, the potential recharge rate ranged between 37–466 mm/a (6–27% of precipitation plus irrigation, P + I) beneath wheat-maize, 110–564 mm/a (12–52% of P + I) beneath orchard, and 7–10 mm/a (1–2% of P + I) beneath woodlands. For the alluvial and lacustrine plain sites, the potential recharge rate ranged between 14–177 mm/a (2–20% of P + I) beneath wheat-maize, 6–57 mm/a (0.5–5% of P + I) beneath orchard, 87–279 mm/a (10–31% of P + I) beneath cotton, and 6–44 mm/a (1–8% of P + I) beneath woodlands. The potential recharge was impacted by various external factors, like lithology, crops, and irrigation. When an irrigation controlled experiment was conducted in the same field with the same crop cultivation, the results revealed that larger irrigation quantities led to larger potential recharge rates.