Abstract
High-resolution data on a field scale is very important for improving our understanding of hydrological processes. This is particularly the case for water-demanding agricultural production systems such as rice paddies, for which water-saving strategies need to be developed. Here we report on the application of an in situ, automatic sampling system for high-resolution data on stable isotopes of water (18O and 2H). We investigate multiple rice-based cropping systems consisting of wet rice, dry rice and maize, with a single, but distributed analytical system on a sub-hourly basis. Results show that under dry conditions, there is a clear and distinguishable crop effect on isotopic composition in groundwater. The least evaporative affected groundwater source is that of maize, followed by both rice varieties. Groundwater is primarily a mixture of irrigation and rainwater, where the main driver is irrigation water during the dry season and rainwater during the wet season. Stable isotopes of groundwater under dry season maize react rapidly on irrigation, indicating preferential flow processes via cracks and deep roots. The groundwater during the dry season under wet and dry rice fields is dominated at the beginning of the growing season mainly by the input of rainwater; later, the groundwater is more and more replenished by irrigation water. Overall, based on our data, we estimate significantly higher evaporation (63–77%) during the dry season as compared to the wet season (27–36%). We also find, for the first time, significant sub-daily isotopic variation in groundwater and surface ponded water, with an isotopic enrichment during the daytime. High correlations with relative humidity and temperature, explain part of this variability. Furthermore, the day-night isotopic difference in surface water is driven by the temperature and relative humidity; however, in groundwater, it is neither driven by these factors.
Highlights
Rice (Oryza sativa L.) is the dominant staple food for nearly half of the world’s population [1], and has the highest water demand and a higher sensitivity to water deficit [2] than other crops [3,4], twice as high than wheat and maize [5]
The system measured data include daytime and nighttime measurements, while lab data were sampled only during daytime (9–11 a.m.). (ii) System data were based on water vapor measurements mixed with dry gas while lab data were based on liquid water measurements. (iii) Lab data were normalized using laboratory standards while we used self-produced standards in the field-sampling system
Measuring the stable isotopes of water using a multi-source and high-frequency system increases the capability of researchers to identify detailed hydrological processes
Summary
Rice (Oryza sativa L.) is the dominant staple food for nearly half of the world’s population [1], and has the highest water demand and a higher sensitivity to water deficit [2] than other crops [3,4], twice as high than wheat and maize [5]. The application of stable isotopes of water as a tool to explore hydrological processes has greatly expanded on spatial and temporal scales [6]. Due to their conservative nature, stable isotopes of water, δ2 H and δ18 O [7] became powerful natural tracers in the hydrological cycle [8,9]. They can be used as a tool for studying hydrological process [10], including climate-driven changes [11], soil hydrological processes [12], and catchment-scale hydrological fluxes [13,14]. Stable isotopes of water in rice grains have been used to identify the geographical origin [15], reconstruction of relative humidity (RH) level [16] and air temperature (T) [17], while stable isotopes of liquid water have been used to estimate the root water uptake [18], evaporation [5,19], and transpiration [20]
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