Maize transpiration and water productivity of two irrigated fields with varying groundwater depths in an arid area

A better understanding of crop transpiration and its influencing factors in arid areas where irrigation is required contributes to improving agricultural water productivity and water resources management. However, previous studies from different regions have not come to a consensus on which factors and how they control crop transpiration, and little is known for areas with a shallow groundwater table. In this study, maize (Zea mays L.) transpiration was investigated under two groundwater table depths (WTDs) during two growing seasons. Sap velocity (SV), micrometeorological conditions and water regimes were monitored to evaluate transpiration rates under different WTDs and their relationships to environmental and hydrologic conditions. Results show that the combination of photosynthetic active radiation (PAR) and vapor pressure deficit (VPD) mainly controlled the diurnal dynamics of SV. PAR better explained the temporal variability of transpiration than solar radiation at hourly scale. At daily scale, the two-segment transpiration response to VPD was observed, suggesting that the studied species likely has a capacity of drought-tolerance. On average, SV was found to be larger during midday for the shallower WTD in 2016 than the deeper one. For different seasons, the shallower WTD in 2017 also led to larger transpiration than the deeper one in 2016. Given similar yields for all cases, the maize field with the deeper WTD in 2016 was estimated to have higher water productivity. An optimum water use efficiency WTD was determined by assessing the normalized transpiration difference with the varying WTD and it was about 2.1–2.2 m for maize in this area. These results shed light on the most effective utilization of the valuable water confronting the changing climate in arid areas with shallow groundwater.