Estimate Cotton Water Consumption from Shallow Groundwater under Different Irrigation Schedules

Shallow groundwater is considered an important water resource to meet crop irrigation demands. However, limited information is available on the application of models to investigate the impact of irrigation schedules on shallow groundwater depth and estimate evaporation while considering the interaction between meteorological factors and the surface soil water content (SWC). Based on the Richards equation, we develop a model to simultaneously estimate crop water consumption of shallow groundwater and determine the optimal irrigation schedule in association with a shallow groundwater depth. A new soil evaporation function was established, and the control factors were calculated by using only the days after sowing. In this study, two irrigation scheduling methods were considered. In Method A, irrigation was managed based on the soil water content; in Method B, irrigation was based on the crop water demand. In comparison with Method B, Method A was more rational because it could use more groundwater, and the ratio of soil evaporation to total evapotranspiration was low. In this model, the interaction between meteorological factors and the SWC was considered to better reflect the real condition; therefore, the model provided a better way to estimate the crop water consumption.

The increasing role of vegetation transpiration in soil moisture loss across China under global warming

Changing pathways of soil moisture loss, either directly from soil (evaporation) or indirectly through vegetation (transpiration), are an indicator of ecosystem and land hydrological cycle responses to the changing climate. Based on the ratio of transpiration to evaporation, this paper investigates soil moisture loss pathway changes across China using five reanalysis-type datasets for the past and Coupled Model Intercomparison Project Phase 6 (CMIP6) climate projections for the future. The results show that across China, the ratio of vegetation transpiration to soil evaporation has generally increased across vegetated land areas, except in grasslands and croplands in North China. During 1981–2014, there was an increase by 51.4 percentage points (pps, p < 0.01) on average according to the reanalyses and by 42.7 pps according to 13 CMIP6 models. The CMIP6 projections suggest that the holistic increasing trend will continue into the 21st century at a rate of 40.8 pps for SSP585, 30.6 pps for SSP245, and –1.0 pps for SSP126 shared socioeconomic pathway scenarios for the period 2015–2100 relative to 1981–2014. Major contributions come from the increases in vegetation transpiration over the semiarid and subhumid grasslands, croplands, and forestlands under the influence of increasing temperatures and prolonged growing seasons (with twin peaks in May and October). The future increasing vegetation transpiration ratio in soil moisture loss implies the potential of regional greening across China under global warming and the risks of intensifying land surface dryness and altering the coupling between soil moisture and climate in regions with water-limited ecosystems.

The Influence of Moss Colonization and Biochar Application on Evaporation Losses and Surface Crack in Shallow Carbonate–Derived Laterite During Dry–Wet Cycles

AimsSoil water deficit in karst mountain lands is becoming an issue of concern owing to porous, fissured, and soluble nature of underlying karst bedrock. It is important to identify feasible methods to facilitate soil water preservation in karst mountainous lands. This study aims to seek the possibility of combined utilization of moss colonization and biochar application to reduce evaporation losses in carbonate-derived laterite.MethodsThe treatments of the experiments at micro-lysimeter included four moss spore amounts (0, 30, 60, and 90 g·m−2) and four biochar application levels (0, 100, 400, and 700 g·m−3). The dynamics of moss coverage, characteristics of soil surface cracks and surface temperature field were identified. An empirical evaporation model considering the interactive effects of moss colonization and biochar application was proposed and assessed.ResultsMoss colonization reduced significantly the ratio of soil desiccation cracks. Relative cumulative evaporation decreased linearly with increasing moss coverage under four biochar application levels. Biochar application reduced critical moss coverage associated with inhibition of evaporation by 33.26%-44.34%. The empirical evaporation model enabled the calculation of soil evaporation losses under moss colonization and biochar application, with the R2 values ranging from 0.94 to 0.99.Conclusions Our result showed that the artificially cultivated moss, which was induced by moss spores and biochar, decreased soil evaporation by reducing soil surface cracks, increasing soil moisture and soil surface temperature.Moss colonization and biochar application has the potential to facilitate soil moisture conservation in karst mountain lands.

Soil related developments of the Biome-BGCMuSo v6.2 terrestrial ecosystem model by integrating crop model components

Terrestrial biogeochemical models are essential tools to quantify climate-carbon cycle feedback and plant-soil relations from local to global scale. In this study, theoretical basis is provided for the latest version of Biome-BGCMuSo biogeochemical model (version 6.2). Biome-BGCMuSo is a branch of the original Biome-BGC model with a large number of developments and structural changes. Earlier model versions performed poorly in terms of soil water content (SWC) dynamics in different environments. Moreover, lack of detailed nitrogen cycle representation was a major limitation of the model. Since problems associated with these internal drivers might influence the final results and parameter estimation, additional structural improvements were necessary. During the developments we took advantage of experiences from the crop modeller community where internal process representation has a long history. In this paper the improved soil hydrology and soil carbon/nitrogen cycle calculation methods are described in detail. Capabilities of the Biome-BGCMuSo v6.2 model are demonstrated via case studies focusing on soil hydrology and soil organic carbon content estimation. Soil hydrology related results are compared to observation data from an experimental lysimeter station. The results indicate improved performance for Biome-BGCMuSo v6.2 compared to v4.0 (explained variance increased from 0.121 to 0.8 for SWC, and from 0.084 to 0.46 for soil evaporation; bias changed from −0.047 to 0.007 m3 m−3 for SWC, and from −0.68 mm day−1 to −0.2 mm day−1 for soil evaporation). Sensitivity analysis and optimization of the decomposition scheme is presented to support practical application of the model. The improved version of Biome-BGCMuSo has the ability to provide more realistic soil hydrology representation and nitrification/denitrification process estimation which represents a major milestone.

Soil Water Stable Isotopes Reveal Surface Soil Evaporation Loss Dynamics in a Subtropical Forest Plantation

Line-conditioned excess (lc-excess), the deviation of the relationship between δD and δ18O in soil water from that of precipitation, is often used to indicate soil evaporation loss, but the conditions of using lc-excess under the influences of precipitation infiltration or percolation had not been identified. The interaction effects of climate, soil and vegetation on soil evaporation in forests are not well known. We collected soil water at 0–5, 15–20 and 40–45 cm depths and event-based precipitation from 2011 to 2015 in a subtropical forest plantation and calculated the lc-excess. Precipitation on the sampling day and percolation of upper soil water with low lc-excess affected the capacity of the lc-excess to indicate the soil evaporation fractionation signals. Lc-excess of soil water at 0–5 cm depth indicated a reliable soil evaporation loss estimate over 30 days prior to the sampling day. Soil evaporation loss was dominated by the cumulative soil temperature (Tss) during drought periods and was dominated by the relative soil water content (RSWC) during non-drought periods. High Tss decreased soil evaporation loss by increasing transpiration and relative humidity. Our results emphasize the importance of sampling the upper-most soil layer when there is no rain and vegetation during drought periods in forests when studying soil evaporation loss dynamics.

Technical note: Evaporating water is different from bulk soil water in <i>δ</i>²H and <i>δ</i>¹⁸O and has implications for evaporation calculation

Soil evaporation is a key process in the water cycle and can be conveniently quantified using δ2H and δ18O in bulk surface soil water (BW). However, recent research shows that soil water in larger pores evaporates first and differs from water in smaller pores in δ2H and δ18O, which disqualifies the quantification of evaporation from BW δ2H and δ18O. We hypothesized that BW had different isotopic compositions from evaporating water (EW). Therefore, our objectives were to test this hypothesis first and then evaluate whether the isotopic difference alters the calculated evaporative water loss. We measured the isotopic composition of soil water during two continuous evaporation periods in a summer maize field. Period I had a duration of 32 d, following a natural precipitation event, and period II lasted 24 d, following an irrigation event with a 2H-enriched water. BW was obtained by cryogenically extracting water from samples of 0–5 cm soil taken every 3 d; EW was derived from condensation water collected every 2 d on a plastic film placed on the soil surface. The results showed that when event water was heavier than pre-event BW, δ2H of BW in period II decreased, with an increase in evaporation time, indicating heavy water evaporation. When event water was lighter than the pre-event BW, δ2H and δ18O of BW in period I and δ18O of BW in period II increased with increasing evaporation time, suggesting light water evaporation. Moreover, relative to BW, EW had significantly smaller δ2H and δ18O in period I and significantly smaller δ18O in period II (p<0.05). These observations suggest that the evaporating water was close to the event water, both of which differed from the bulk soil water. Furthermore, the event water might be in larger pores from which evaporation takes precedence. The soil evaporative water losses derived from EW isotopes were compared with those from BW. With a small isotopic difference between EW and BW, the evaporative water losses in the soil did not differ significantly (p>0.05). Our results have important implications for quantifying evaporation processes using water stable isotopes. Future studies are needed to investigate how soil water isotopes partition differently between pores in soils with different pore size distributions and how this might affect soil evaporation estimation.

Transpiration and Water Use of an Irrigated Traditional Olive Grove with Sap-Flow Observations and the FAO56 Dual Crop Coefficient Approach

The SIMDualKc model was applied to evaluate the crop water use and the crop coefficient (Kc) of an irrigated olive grove (Olea europaea L.) located in Sicily, Italy, using experimental data collected from two crop seasons. The model applies the FAO56 dual Kc approach to compute the actual crop evapotranspiration (ETc act) and its components, i.e., the actual tree transpiration (Tc act), obtained through the basal crop coefficient (Kcb), and soil evaporation according to an evaporation coefficient (Ke). Model calibration was performed by minimizing the difference between the predicted Tc act and the observed daily tree transpiration measured with sap flow instrumentation (TSF field) acquired in 2009. The validation was performed using the independent data set of sap flow measurements from 2011. The calibrated Kcb was equal to 0.30 for the initial and non-growing season stages, 0.42 for the mid-season, and 0.37 for the end season. For both seasons, the goodness-of-fit indicators relative to comparing TSF field with the simulated Tc act resulted in root mean square errors (RMSE) lower than 0.27 mm d−1 and a slope of the linear regression close to 1.0 (0.94 ≤ b0 ≤ 1.00). The olive grove water balance simulated with SIMDualKc produced a ratio between soil evaporation (Es) and ETc act that averaged 39%. The ratio between actual (ETc act) and potential crop evapotranspiration (ETc) varied from 84% to about 99% in the mid-season, indicating that the values of ETc act are close to ETc, i.e., the adopted deficit irrigation led to limited water stress. The results confirm the suitability of the SIMDualKc model to apply the FAO56 dual Kc approach to tree crops, thus assessing the water use of olives and supporting the development of appropriate irrigation management tools that are usable by farmers. A different way to estimate Kcb is based on the approach suggested in 2009 by Allen and Pereira (A&P), which involves the measured fraction of ground covered (shaded) by the crop and the height of the trees. Its application to the studied grove produced the mid-season Kcb values ranging from 0.40–0.45 and end-season Kcb values ranging from 0.35–0.40. The comparison between the A&P-computed Tc act A&P and TSF field shows RMSE values ranging from 0.27 to 0.43 mm d−1, which demonstrates the adequacy of the latter approach for parameterizing water balance models and for irrigation scheduling decision making.

Improved Runoff Simulations for a Highly Varying Soil Depth and Complex Terrain Watershed in the Loess Plateau with the Community Land Model

This study aimed to improve runoff simulations and explore deep soil hydrological processes for a watershed in the center of the Loess Plateau (LP), China. This watershed, the Wuding River Basin (WRB), has very complex topography, with soil depths ranging from 0 to 197 m. The hydrological model used for our simulations was the Community Land Model (CLM) version 5 developed by the National Center for Atmospheric Research. Actual soil depths and river channels were incorporated into CLM to realistically represent the physical features of the WRB. Through sensitivity tests, CLM with 150 soil layers produced the most reasonable results and was adopted for this study. Our results showed that CLM with actual soil depths significantly suppressed unrealistic variations of the simulated sub-surface runoff when compared to the default simulations with a fixed soil depth of 8 m. In addition, CLM with higher-resolution soil layering slightly improved runoff simulations, but generated simulations with much smoother vertical water flows that were consistent with the uniform distribution of soil textures in our study watershed. The runoff simulations were further improved by the addition of river channels to CLM, where the seasonal variability of the simulated runoff was reasonably captured. Moreover, the magnitude of the simulated runoff remarkably decreased with increased soil evaporation by lowering the soil water content threshold, which triggers surface resistance. The lowered threshold was consistent with the loess soil, which has a high sand component. Such soils often generate stronger soil evaporation than soils dominated by clay. Finally, with the above changes in CLM, the simulated total runoff matched very closely with observations. When compared with those for the default runoff simulations, the correlation coefficient, root-mean-square error, and Nash Sutcliffe coefficient for the improved simulations changed dramatically from 0.02, 10.37 mm, and −12.34 to 0.62, 1.8 mm, and 0.61. The results in this study provide strong physical insight for further investigation of hydrological processes in complex terrain with deep soils.