Estimating water flux and evaporation losses using stable isotopes of soil water from irrigated agricultural crops in tropical humid regions

Abstract

Abstract. Reliable information on water flow dynamics and water losses via irrigation on irrigated agricultural fields is important to advance water management strategies. We investigated the effect of season (wet season, dry season), irrigation management (flooded, non-flooded), and crop diversification (wet rice, dry rice, and maize) on soil water dynamics and water losses via evaporation during plant growth. Soil water was extracted and analyzed for the stable isotopes of water (δ2H and δ18O), and the fraction of evaporation loss was determined using the Craig–Gorden equation. For all crops, shallow soil compartments (0 to 0.2 m) were more isotopically enriched than deep soils (below 0.2 m). The soil in maize fields showed stronger evaporation enrichment than rice, which increased as the crops progressed through the growth; however, it decreased in both rice varieties during both seasons. Greater water loss was encountered during the wet season even though evaporation signals were stronger during the dry season. The enrichment of surface water was reflected in shallow soils of wet rice, and it decreased towards the end of growth during the wet and dry seasons. Isotope data indicated that soil water flow mechanisms varied depending on field conditions. In flooded conditions, surface soil was consistently affected by piston type matrix flow. During non-flooded conditions, matrix flow via diffusion dominated compared to upwards evaporative flux. Occasionally, preferential flows occurred through cracks, especially in maize fields. In wet rice fields, soil water was largely influenced by short-term variability of precipitation events during the wet season and subsequent formation of hydrogen compounds as a result of continued wetness and anaerobic physiochemical conditions that depleted δ2H with respect to δ18O.

Using Soil Water Stable Isotopes to Investigate Soil Water Movement in a Water Conservation Forest in Hani Terrace

“…Vegetation types therefore have a dominant impact on soil water movement. Amani et al [63] also confirmed the influence of vegetation and field conditions on the variability of soil water flow mechanisms. In addition, the slope and intercept of the SWL in the wild grassland were higher than those of the arbor and shrub forests; this suggests that precipitation was the dominant supply of soil water in the grassland, while early soil water predominantly replenished soil water in the arbor land and shrubland.…”

Deep soil water <sup>18</sup>O and <sup>2</sup>H measurements preserve long term evaporation rates on China's Loess Plateau

“…As the lc-excess is a more representative 280 indicator of evaporation than the single isotope system ( 8 O or 2 H) and the d-excess (Sprenger et al, 2017b;Masson-Delmotte et al, 2005), evaporation estimates from the new method should be more accurate. Moreover, similar to the d-excess method (Hu et al, 2018), the new method combines 2 H and 18 O, avoiding the inconsistency between the separate implementation of 2 H and 18 O in the Craig-Gordon method (Sprenger et al, 2017a;Mahindawansha et al, 2019). Overall, f estimation using the new method is more robust than the remaining two methods and would be widely 285 applicable.…”

“…Conversely, a relatively long-term f can be estimated using stable 2 H and 18 O isotopes, because they are only enriched dur-40 ing soil evaporation processes (Zimmermann et al, 1966) and can integrate soil evaporation processes occurring over interest of period (Jasechko et al, 2013;Allison and Barnes, 1983). The primary advantage is that it does not need continuous field observations with high frequency and have been used to estimate f in various studies (Allison and Barnes, 1983;Sprenger et al, 2017a;Hsieh et al, 1998;Mahindawansha et al, 2019). Previous studies commonly utilize the shallow soil water 2 H / 18 O isotopes; however, obtaining long-term average f requires continuous sampling effort of shallow soil 45 due to the strong isotopic dynamics in that part of the soil profile, which can be costly.…”

“…Methods to determine f from isotopic composition of water are generally based on the well-known Craig-Gordon model by single-isotope system ( 18 O or 2 H) (Craig and Gordon, 1965); however, estimates from 18 O are often inconsistent with those from 2 H (Sprenger et al, 2017a;Mahindawansha et al, 2019). The dual-isotope systems, including the deuterium-excess 55 (d-excess, as defined by Dansgaard (1964)) and the line-conditioned excess (lc-excess, Landwehr and Coplen (2006)), overcome the inconsistency, but are often used as a qualitative indicator of evaporation intensity rather than quantitatively.…”

An Improved Craig–Gordon Isotopic Model: Accounting for Transpiration Effects on the Isotopic Composition of Residual Water during Evapotranspiration

“…The Craig-Gordon model (C-G model), proposed by Harmon Craig and Louis Gordon [15], is a renowned isotopic model that explains the combined effect of equilibrium and kinetic isotopic fractionation during the phase transition from liquid water to vapor [16][17][18]. This model is commonly used to interpret and model the isotopic composition of oceans [19], lakes, atmospheric water, soil water, and other natural water systems [8,20,21]. It has become an important theoretical method for isotopic studies of the hydrological cycle, both regionally and globally.…”