High-Frequency Water Isotopic Analysis Using an Automatic Water Sampling System in Rice-Based Cropping Systems

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.

High-Resolution, In Situ Monitoring of Stable Isotopes of Water Revealed Insight into Hydrological Response Behavior

“…A custom-made, automated mobile laboratory, the Water Analysis Trailer for Environmental Research (WATER), was utilized to automatically sample multiple water sources (e.g., stream water, groundwater, and precipitation) and analyze the stable isotopes of water (δ 18 O and δ 2 H) and water chemistry in situ ( Figure 2). Constructing the trailer was based on previous experiences made by Heinz et al [14] and Mahindawansha et al [15]. Figure 3 illustrates the technical set up of the WATER.…”

“…Manual high-resolution, multiple-source sampling is restricted to short durations and might fail in the sampling of relevant events due to their erratic and difficult-to-plan nature. Therefore, a few groups developed and utilized automatic sampling and analytical systems for in situ monitoring recently to overcome these problems [13][14][15]. Such systems allow gaining insight into previously unknown hydrological dynamics.…”

“…Heinz et al [14] presented the technical setup of an automated sampling system and provided an initial proof-of-concept to monitor multiple water sources (surface water and groundwater) in up to ten rice paddies of a field trial in the Philippines. Building on this system, Mahindawansha et al [15] investigated seasonal and crop effects on isotopic compositions of surface water and groundwater in these rice paddies. An important, yet poorly known mechanism is the response of several water sources in real catchments, including that of shallow groundwater to precipitation.…”

Isotopes in Hydrology and Hydrogeology

“…Nine articles [3][4][5][6][7][8][9][10][11] look at the start of the hydrological cycle and its processes (including lake water balance), and at the origin of rainfall and its selection in groundwater recharge using characteristic, naturally occurring δ 2 H and δ 18 O signatures. Spatial and temporal modifications in hydrogen and oxygen isotopic compositions (δ 2 H and δ 18 O) could be associated to equilibrium exchanges occurring at all phase transitions and kinetic isotopic fractionation during the transformation from the ocean surface to the atmosphere, small rainfall amounts when raindrops fall below the cloud [3].…”

“…In particular, based on the data of water stable isotopes (δ 2 H and δ 18 O) in the Naqu River basin (China), Chen et al [3] developed a framework of the variable fuzzy evaluation model (VFEM) to provide a method to classify stable isotopes and to generalize the source identification of water replenishment by rainfall or snowmelt in a river basin. Mahindawansha et al [4] propose an automatic sampling system for high-resolution data on stable isotopes of water (δ 2 H and δ 18 O) to investigate multiple rice-based cropping systems in Los Baños, Laguna (Philippines). Results show that, under dry conditions, there is a clear and distinguishable crop effect on isotopic composition in groundwater.…”