Initial Soil Water Research Articles

Funções de estimativa do armazenamento de água no solo sob diferentes níveis de capacidade de água disponível

The estimative of soil water storage is essential for representation of soil water relations for Brazil agricultural planning purposes. We had to aim evaluate the performance of five equations used to estimate soil water storage, through simulations of a daily water balance to the cities of Goiânia-GO and Ponta Grossa-PR, considering different amounts of total available water in the soil. For that purpose, were used daily data series of precipitation, reference evapotranspiration, crop coefficient, p fraction and initial soil water storage. It was considered total available water from 20 mm to 320 mm, with an interval of 30 mm. Potential functions of First and Second Order were approximately equal to the Cosine function in a daily water balance simulations, under conditions of supplementary irrigation and non-irrigation over the year. The Exponential function was not adequate in the estimative of soil water storage. Braga function was not adequate when considering additional irrigations in a daily water balance. We recommend the improvement of the functions to estimate soil water storage from replacement of “negative cumulative” function by other parameters, which could better explain the variation of soil water storage available for plants.

Stratification response of soil water content during rainfall events under different rainfall patterns

AbstractMany researchers have studied the influence of rainfall patterns on soil water movement processes using rainfall simulation experiments. However, less attention has been paid to the influence under natural condition. In this paper, rainfall, soil water content (SWC), and soil temperature at 10‐, 20‐, 30‐, 40‐, and 50‐cm depths were simultaneously monitored at 1‐min intervals to measure the variation in SWC (SWCv) in response to rainfall under different rainfall patterns. First, we classified rainfall events into four patterns. During the study period, the main pattern was the advanced rainfall pattern (38% of all rainfall events), whereas the delayed, central, and uniform rainfall patterns had similar frequencies of about 20%. During natural rainfall, rainwater rapidly passed through the top soil layers (10–40 cm) and was accumulated in the bottom layer (50 cm). When a high rainfall pulse occurred, the water storage balance was disturbed, resulting in the drainage of initial soil water from the top layers into the deeper layers. Therefore, the critical function of the top layers and the bottom layers was infiltration and storage, respectively. The source of water stored in the bottom layer was not only rainfall but also the initial soil water in the upper soil layers. Changes in soil temperature at each soil depth were comonitored with SWCv to determine the movement characteristics of soil water under different rainfall patterns. Under the delayed rainfall pattern, preferential flows preferred to occur. Under the other rainfall patterns, matrix flow was the main form of soil water movement. Rainfall amount was a better indicator than rainfall intensity for SWCv in the bottom layer under the delayed rainfall pattern. These results provide insights into the responses of SWCv under different rainfall patterns in northern China.

Assessing spring maize responses to irrigation and nitrogen regimes in north-west India using CERES-Maize model

Inadequate availability of fresh water and rising cost of fertilizers warrant their judicious use in agriculture. In intensively-cropped Punjab state of north-west India, alarming decline in groundwater resources pose a challenge to spring maize that otherwise has high profitability. This study is an assessment of water use and productivity responses of spring maize to irrigation and nitrogen regimes in a subtropical environment using CERES-Maize model. Database was generated from a field study on maize planted in second half of February with combinations of two irrigation regimes viz., irrigation water to pan evaporation ratio of 1.0 (I1.0) and 0.5 (I0.5), and four N rates viz., 0, 50, 100 and 150 kg ha−1 on a sandy loam soil. Irrigation and N had significant effects on grain yield, water use and N uptake. Performance of the model was reasonable with normalized root mean square of deviations between simulated and measured values less than 20% for harvest-time biomass, grain yield and water use; and slightly greater variance (30%) for grain N uptake. Scenario analysis showed that ET-based water productivity (WPET) was greater in January 31 than February 14 planted maize crop. The WPET was greater with I0.5 than with I1.0 suggesting that increase in yield was less than proportional increase in ET. The WPET responses to N with increase in irrigation were greater at lower than at higher initial soil water.

An efficient analytical model for horizontal infiltration in soils

An efficient method for nonlinear approximate solutions of the one-dimensional horizontal absorption equation is presented. The method pertains to a semi-infinite soil with uniform initial condition and constant soil water content at the beginning of a soil column. It does not make any prior assumptions about the form of the diffusivity function. The nonlinear solution is of simple combination of power and exponential functions and expresses in explicit form the distance as function of time and soil water content. It involves all the physical parameters and four additional fitting parameters which can be obtained by simple fitting procedure. Algebraic expressions that quantify the main infiltration process are derived from the basic equation. These include the sorptivity, the infiltration rate, the cumulative infiltration rate, and the wetting front. A comparison with exact, numerical and other analytical solutions validates the accuracy of the approximation and shows its advantage to other approximate solutions. The analytical expressions are used to predict experimental soil moisture profiles from simple measurements such as sorptivity, wetting front, and initial and final soil water contents. The method is applied to estimate soil hydraulic properties using horizontal water absorption experiments. The results of the parameter estimation procedure are compared to other existing results and show good agreements. The solution method is also tested to various diffusivity functions and its limitation is discussed in detail.

Putah Creek hydrology affecting riparian cottonwood and willow tree survival.

Creating or recruiting new riparian forests to improve Lower Putah Creek (LPC) ecosystem functions is challenging under the modified stream flow regime developed after historic gravel mining and installation of Monticello Dam upstream. Hydrologic connectivity between riparian trees, shallow groundwater, and the low flow channel is essential towards maintaining these forests and related habitats through the annual summer and multi-year drought periods typical in this Sacramento Valley region of California. Despite increased average summer flows, significant mature cottonwood and willow tree mortality along the LPC riparian areas below the Putah Creek diversion dam in 2014 raised concerns over the soil and hydrologic factors affecting riparian vegetation survival. A forensic analysis was conducted combining annual canopy coverage fractions and tree ring studies with daily soil-water balances, low flow records, and available groundwater level Information from the past few decades to determine the key hydrologic factors affecting riparian tree survival along the LPC. The 2011-2016 drought was linked with greater than prior average soil-water deficits in 2012-2015 and lower initial soil-water storage on March 1 of 2012 and 2014 that would be expected to stress the trees. However, such stress was not apparent in decreased tree ring spacing during this period from mature (40-50years old), deceased, and living trees. Tree canopy coverage declined dramatically (by as much as 50% as compared to the previous decade average) only in the summer of the 2014 despite a ~ 35% increase in average summer flows from 2011 to 2014. However, the regional water table aquifer levels declined at an average rate of ~ 35mm/day in 2014 (as compared to ~ 17mm/day in previous decade) and by several meters overall between 2011 and 2016 suggesting that deceased trees lacked access to the water table aquifer or lateral stream seepage. The increased rates of water table decline and overall depth may be associated with a large increase in adjacent irrigated almond orchard areas in 2014-2016. Knowledge of the dynamic hydrologic factors controlling sustainability of riparian trees should better inform and guide future tree restoration efforts along the LPC.

The success story of irrigation against salinity in Violada, NE Spain

AbstractIn the early 20th century, when extensive areas of land were transformed from rainfed to irrigated agriculture, there was a severe lack of scientific and technical literature available on the effects of applying water to saline soils. This was the case of the Violada Irrigation District located in a semiarid region of NE Spain whose transformation started in the 1940s. We discuss the use of agronomical expertise and the limited knowledge of soils available prior to the transformation, the favorable and unfavorable scenarios encountered during the transformation, and the final success of irrigation despite the initial soil salinization and water logging. We attribute the technical success to the fact that gypsum is common in the soils and geological materials, and to the continued drainage efforts. Failures in the irrigation of saline lands are a frequent subject of discussion in the scientific literature; contrariwise, we present the history of a successfully irrigated district after 70 years.

Optimization of irrigation scheduling for spring wheat based on simulation-optimization model under uncertainty

Water scarcity is the major constraint to social-economic development in arid and semiarid regions, where irrigation needs to be scheduled properly for the main crops. In this study, a simulation-optimization model for crop optimal irrigation scheduling under uncertainty was developed to maximize the net benefit. The model integrated a water-driven crop model (AquaCrop) with the optimization model, and incorporated the generation technique for the interval values of hydrological parameters (i.e., precipitation and evapotranspiration) and crop market prices to deal with uncertainties in these variables. The water price was assumed constant. The model was calibrated based on field experimental data obtained in 2014 and validated using 2015 data. The field experiments involved spring wheat (Yongliang No. 4) at Shiyang River Basin Experiment Station in Wuwei City, Gansu Province of Northwest China. The model was then used to generate the optimal irrigation schedules under various irrigation amounts, irrigation events, initial soil water storage and crop market price under uncertainty. Results indicated that the model is applicable for reflecting the complexities of simulation-optimization under uncertainties for spring wheat irrigation water scheduling. The optimization results indicated that the optimal irrigation amount range was [185, 322] mm with the corresponding optimal net benefit of [1.05, 2.77] ×104 Yuan/hm2 and yield of [7.4, 7.6] kg/hm2 for extreme wet conditions in the basin (defined as the combination of the 5% frequency precipitation with 95% frequency evapotranspiration). For extreme dry conditions, the optimal irrigation amount range was [442, 507] mm with the optimal net benefit of [0.85, 2.64] ×104 Yuan/hm2 and the corresponding yield of [6.6, 7.4] kg/hm2. Results also showed that four irrigation events under higher initial soil water storage were more likely to produce the higher net benefit and the optimal net benefit would increase as the crop market price increases, as expected. This work can be used to guide irrigation management for local farmers.

Numerical Modeling of Water Movement from Buried Vertical Ceramic Pipes through Soils

The problem of water scarcity is becoming common in many parts of the world, to overcome part of this problem proper management of water and an efficient irrigation system are needed. Irrigation with a buried vertical ceramic pipe is known as a very effective in the management of irrigation water. The two- dimensional transient flow of water from a buried vertical ceramic pipe through homogenous porous media is simulated numerically using the HYDRUS/2D software. Different values of pipe lengths and hydraulic conductivity were selected. In addition, different values of initial volumetric soil water content were assumed in this simulation as initial conditions. Different values of the applied head were assumed in this simulation as boundary conditions. The results of this research showed that greater spreading occurs in the horizontal direction. Increasing applied heads, initial soil water contents and pipe hydraulic conductivities, cause increasing the size of wetting patterns but in a few increases. Also, the results showed that the empirical formulas which can be used for expressing the wetted width and depth in terms of applied head, initial soil water content, application time, pipe hydraulic conductivity, and pipe length, are good and can be used as design equations. 

Sequence of Changes in Maize Responding to Soil Water Deficit and Related Critical Thresholds.

The sequence of changes in crop responding to soil water deficit and related critical thresholds are essential for better drought damage classification and drought monitoring indicators. This study was aimed to investigate the critical thresholds of maize growth and physiological characteristics responding to changing soil water and to reveal the sequence of changes in maize responding to soil water deficit both in seedling and jointing stages based on 2-year’s maize field experiment responding to six initial soil water statuses conducted in 2013 and 2014. Normal distribution tolerance limits were newly adopted to identify critical thresholds of maize growth and physiological characteristics to a wide range of soil water status. The results showed that in both stages maize growth characteristics related to plant water status [stem moisture content (SMC) and leaf moisture content (LMC)], leaf gas exchange [net photosynthetic rate (Pn), transpiration rate (Tr), and stomatal conductance (Gs)], and leaf area were sensitive to soil water deficit, while biomass-related characteristics were less sensitive. Under the concurrent weather conditions and agronomic managements, the critical soil water thresholds in terms of relative soil moisture of 0–30 cm depth (RSM) of maize SMC, LMC, net Pn, Tr, Gs, and leaf area were 72, 65, 62, 60, 58, and 46%, respectively, in seedling stage, and 64, 64, 51, 53, 48, and 46%, respectively, in jointing stage. It indicated that there is a sequence of changes in maize responding to soil water deficit, i.e., their response sequences as soil water deficit intensified: SMC ≥ LMC > leaf gas exchange > leaf area in both stages. This sequence of changes in maize responding to soil water deficit and related critical thresholds may be better indicators of damage classification and drought monitoring.

Precipitation events reduce soil respiration in a coastal wetland based on four-year continuous field measurements

Coastal wetlands are considered as a significant sink for global carbon because their organic-rich soils. Given exposed to shallow water tables, water from groundwater is transported upward to the root zone through capillary rise, thus soil moisture in coastal wetlands is relatively high even when there is no precipitation. We expected that as precipitation occurred, the soils in coastal wetlands might become quickly saturated and lead to the development of anoxic conditions. We further hypothesized that such anoxic conditions might decrease soil respiration by limiting oxygen availability and biological activities of roots and microorganisms. Based on continuous automated soil respiration data collected in a coastal wetland in the Yellow River Delta over 4 years (2012–2015), the results showed that on the annual scale, cumulative soil respiration was 317, 321, 231, and 274 g C m−2 yr-1 for 2012, 2013, 2014, and 2015, respectively, with an average of 286 g C m−2 yr-1. The rate of soil respiration increased exponentially with soil temperature during each year and its two seasons (growing season and non-growing season). In addition, soil respiration was significantly related to soil moisture during the growing season, but was not affected by soil moisture during the non-growing season. After each precipitation event, soil respiration was significantly negatively correlated with soil moisture under different initial soil water contents. There was a significant positive correlation between changes in soil respiration and changes in soil moisture following precipitation events. Moreover, the increase of soil moisture following precipitation events changed the temperature response of soil respiration. Our study indicated that precipitation events could decrease soil respiration by increasing soil moisture and inducing anoxic conditions in the coastal wetland. Therefore, we speculate that the continuation of decreasing precipitation and increasing temperature trends in the Yellow River Delta may increase soil carbon losses in the coastal wetland due to the increase in soil respiration.

Testing a crop model with extreme low yields from historical district records

Extreme weather events across the world cause large variations in daily seasonal temperature and precipitation, potentially reducing grain yield and negatively affecting global food security. Thus, it is important to assess if crop growth models can simulate the yield losses caused by these extreme weather events. We tested if the DSSAT-NWheat crop simulation model, which has been successfully validated with many controlled field-experimental data across the world, can reproduce historical extreme low-yielding years at three global locations in the USA, France, and Australia. The crop model reproduced extreme low yields in some of the driest and hottest years at all three locations. However, the model failed to simulate some of the observed extreme low-yielding years. Also, the model simulated extreme low yields in some years that were not extreme low yielding in the observed records. Surprisingly, some of the driest and hottest years did not show up as observed extreme low-yielding years in the district records. This discrepancy could be explained by reporting yields, or in this case “not reporting extreme low yields” in district records, as indicated by large drops in harvested areas in extreme dry and hot seasons. Other discrepancies exist because crop models do not often consider many factors under farmer-field conditions that lead to extreme low yields in district records, including frost, hail and lodging, pests and diseases, and excess water. Historical district yield records are also limited for model testing due to unknown and changing aggregation across a district, possible omission of extreme low-yielding fields in some years, and unknown spatial and temporal varying cultivars and crop management, particularly in initial soil water conditions. In conclusion, we do not recommend using historical district yield records for model testing of extreme low yields.

Estimation of irrigation requirements for drip-irrigated maize in a sub-humid climate

Drip-irrigation is increasingly applied in maize (Zea mays L.) production in sub-humid region. It is critical to quantify irrigation requirements during different growth stages under diverse climatic conditions. In this study, the Hybrid-Maize model was calibrated and applied in a sub-humid Heilongjiang Province in Northeast China to estimate irrigation requirements for drip-irrigated maize during different crop physiological development stages and under diverse agro-climatic conditions. Using dimensionless scales, the whole growing season of maize was divided into diverse development stages from planting to maturity. Drip-irrigation dates and irrigation amounts in each irrigation event were simulated and summarized in 30-year simulation from 1981 to 2010. The maize harvest area of Heilongjiang Province was divided into 10 agro-climatic zones based on growing degree days, arid index, and temperature seasonality. The simulated results indicated that seasonal irrigation requirements and water stress during different growth stages were highly related to initial soil water content and distribution of seasonal precipitation. In the experimental site, the average irrigation amounts and times ranged from 48 to 150 mm with initial soil water content decreasing from 100 to 20% of the maximum soil available water. Additionally, the earliest drip-irrigation event might occur during 3- to 8-leaf stage. The water stress could occur at any growth stages of maize, even in wet years with abundant total seasonal rainfall but poor distribution. And over 50% of grain yield loss could be caused by extended water stress during the kernel setting window and grain filling period. It is estimated that more than 94% of the maize harvested area in Heilongjiang Province needs to be irrigated although the yield increase varied (0 to 109%) in diverse agro-climatic zones. Consequently, at least 14% of more maize production could be achieved through drip-irrigation systems in Heilongjiang Province compared to rainfed conditions.

An evaluation of a methodology for seasonal soil water forecasting for Australian dry land cropping systems

Soil water is a critical resource in many rain-fed agricultural systems. Climate variability represents a significant risk in these systems, which has been addressed in the past through seasonal weather outlooks. This study undertakes a pilot assessment of the potential to extend seasonal weather outlooks to plant available soil water (PASW). We analyse 20 sites in the southeast Australian wheat belt using seasonal weather outlooks from the Predictive Ocean-Atmosphere Model for Australia (POAMA; (the operational seasonal model of the Australian Bureau of Meteorology), which were downscaled and used in conjunction with the Agricultural Production Simulator (APSIM). Hindcast rainfall, potential evapotranspiration (PET) and PASW outlooks were produced on a monthly basis for 33 years at a point scale. The outlooks were assessed using a range of ensemble verification tools. The results showed hit rates that outperformed climatology for rainfall and PET in the short-term (0–2 months), and for PASW with longer lead times (2–5 months). Continuous rank probability skill scores (CRPSS) were generally statistically worse than climatology for rainfall and PET and statistically better than climatology for PASW over 1–3 months. The influence of initial soil water is seasonally dependent, with longer dependence in low evapotranspiration periods. Improved weather model downscaling approaches would transition to climatology and could improve both weather and PASW outlooks. PASW outlooks were strongly reliant on initial conditions, indicating the importance of understanding current soil water status, which needs to be interpreted in a seasonal context as its influence varies over the year. Expanded operational soil water monitoring would be important if PASW outlooks are to become routine.

Effect of soil properties on Hydraulic characteristics under subsurface drip irrigation

Subsurface drip irrigation (SDI) is a technique that has a high potential in application because of its high efficiency in water-saving. The hydraulic characteristics of SDI sub-unit pipe network can be affected by soil physical properties as the emitters are buried in soils. The related research, however, is not fully explored. The laboratory tests were carried out in the present study to determine the effects of hydraulic factors including operating pressure, initial soil water content, and bulk density on flow rate and its sensitivity to each hydraulic factor for two types of SDI emitters (PLASSIM emitter and Heping emitter). For this purpose, three soils with contrasting textures (i.e., light sand, silt loam, and light clay) were repacked with two soil bulk density (1.25 and1.40 g cm-3) with two initial soil water content (12% and 18%) in plexiglass columns with 40 cm in diameter and 40 cm in height. Drip emitters were buried at depth of 20 cm to measure the flow rates under seven operating pressures (60, 100, 150, 200, 250, 300, and 370 kPa). We found that the operating pressure was the dominating factor of flow rate of the SDI emitter, and flow rate increased with the increase of operating pressure. The initial soil water content and bulk density also affected the flow rate, and their effects were the most notable in the light sand soil. The sensitivity of flow rate to each hydraulic factor was dependent on soil texture, and followed a descending order of light sand>silt loam>light clay for both types of emitters. Further, the sensitivity of flow rate to each hydraulic factor decreased with the increase of operating pressure, initial soil water content, and bulk density. This study may be used to guide the soil specific-design of SDI emitters for optimal water use and management.

A Simplified Infiltration Model for Predicting Cumulative Infiltration during Vertical Line Source Irrigation

Vertical line source irrigation is a water-saving irrigation method for enhancing direct water and nutrient delivery to the root zone, reducing soil evaporation and improving water and nutrient use efficiency. To identify its influencing factors, we performed computer simulations using the HYDRUS-2D software. The results indicate that for a given soil, the line source seepage area, but not the initial soil water content and buried depth, has a significant effect on the cumulative infiltration. We thus proposed a simplified method, taking into account the seepage area for predicting the cumulative infiltration based on the Philip model. Finally, we evaluated the accuracy of the simplified method using experimental data and found the cumulative infiltrations predicted by the simplified method were in very good agreement with the observed values, showing a low mean average error of 0.028–0.480 L, a root mean square error of 0.043–0.908 L, a percentage bias of 0.321–0.900 and a large Nash-Sutcliffe coefficient close to 1.0 (NSE ≥ 0.995). The results indicate that this simplified infiltration model, for which the only emitter parameter required is the seepage area, could provide a valuable and practical tool for irrigation design.

Land use dependent variation of soil water infiltration characteristics and their scale-specific controls

Land use change has remarkable impact on hydrological processes. However, little is known on the land use dependent variation of soil water infiltration characteristics. This study aims to investigate the effects of land use type on the spatial variation of infiltration characteristics and to identify their scale-specific controls. A total of 132 in-situ infiltration measurements were conducted using a disc infiltrometer along four 96-m transects under different land use types (forestland, shrubland, cropland with tillage, and cropland without tillage) in a typical agricultural region in north China. Classical statistics and multivariate empirical mode decomposition (MEMD) were used to explore the features of the overall and scale-specific spatial variation of four infiltration characteristics, i.e., cumulative infiltration over 30 min (I30), sorptivity (S), unsaturated hydraulic conductivity (K0, h0 = –2 cm) and saturated hydraulic conductivity (Ks). Besides, the scale-specific correlations between the infiltration characteristics and selected environmental factors, i.e., bulk density (BD), soil organic matter (SOM), initial soil water content (IWC) and water temperature (T) were investigated. The results indicated that land use type significantly affected not only the values of the infiltration characteristics but also their overall and scale-specific spatial variability. Tillage was found to have great impact on these infiltration characteristics. Generally, cropland with tillage revealed significantly higher values and weaker overall spatial variation of I30 and S; while its K0 and Ks were significantly lower. Four major common spatial scales, i.e., 8–9 m, 13–16 m, 28–34 m and 47–94 m, were identified for the infiltration characteristics under different land use types using MEMD. The spatial variation of infiltration characteristics in forestland and shrubland were decomposed into larger common scales than that in croplands. Furthermore, the dominating factors of infiltration characteristics were scale-specific and also varied with land use type. Generally, SOM and BD were considered as important controls in smaller scales in croplands and the impacts of IWC and T were more effective in larger scales in forestland and shrubland.

Soil aggregate breakdown in a field experiment with different rainfall intensities and initial soil water contents

SummaryThe breakdown of soil aggregates under the impact of rainfall is an important process in water erosion of agricultural soil. It determines the size characteristics of the disintegrated fragments and thus their availability for size‐selective sediment transport and deposition. However, little is known about the dynamics of aggregate breakdown and its dependence on soil and rainfall properties under field conditions. In this research, we carried out artificial rainfall experiments to quantify the dynamics of aggregate breakdown in an agricultural field at two levels each of rainfall intensity (RI) and initial soil water content (IWC). In all treatments, the temporal dynamics of aggregate breakdown followed the same two‐stage pattern: a steep exponential decrease in fragment mean weight diameter (MWD) during the first few millimetres of rainfall continued as a slow linear decrease at the second stage. The fragment mass size distribution (FSD) at the end of the rainfall events differed significantly among treatments. Slaking appeared to be the dominant breakdown mechanism in initially dry soil and was particularly strong at the higher RI, whereas mechanical breakdown was more prevalent in prewetted soil with no clear dependence on RI. We propose an empirical model that describes the effect of RI, IWC and initial aggregate stability on the decrease in MWD during the two stages of the breakdown process. The calibrated model successfully predicted the final MWDs in additional experiments, showing its potential for incorporation into erosion models to predict the size characteristics of surface soil fragments resulting from rainfall‐induced aggregate breakdown for various initial soil moisture conditions.Highlights Rainfall‐induced aggregate breakdown was investigated under field conditions. A two‐stage breakdown pattern was observed in all treatments. Slaking appeared to be the dominant mechanism at the first stage, and mechanical breakdown at the second. A simple empirical model gave good predictions of aggregate fragmentation.

Process-based modelling of NH<sub>3</sub> exchange with grazed grasslands

Abstract. In this study the GAG model, a process-based ammonia (NH3) emission model for urine patches, was extended and applied for the field scale. The new model (GAG_field) was tested over two modelling periods, for which micrometeorological NH3 flux data were available. Acknowledging uncertainties in the measurements, the model was able to simulate the main features of the observed fluxes. The temporal evolution of the simulated NH3 exchange flux was found to be dominated by NH3 emission from the urine patches, offset by simultaneous NH3 deposition to areas of the field not affected by urine. The simulations show how NH3 fluxes over a grazed field in a given day can be affected by urine patches deposited several days earlier, linked to the interaction of volatilization processes with soil pH dynamics. Sensitivity analysis showed that GAG_field was more sensitive to soil buffering capacity (β), field capacity (θfc) and permanent wilting point (θpwp) than the patch-scale model. The reason for these different sensitivities is dual. Firstly, the difference originates from the different scales. Secondly, the difference can be explained by the different initial soil pH and physical properties, which determine the maximum volume of urine that can be stored in the NH3 source layer. It was found that in the case of urine patches with a higher initial soil pH and higher initial soil water content, the sensitivity of NH3 exchange to β was stronger. Also, in the case of a higher initial soil water content, NH3 exchange was more sensitive to the changes in θfc and θpwp. The sensitivity analysis showed that the nitrogen content of urine (cN) is associated with high uncertainty in the simulated fluxes. However, model experiments based on cN values randomized from an estimated statistical distribution indicated that this uncertainty is considerably smaller in practice. Finally, GAG_field was tested with a constant soil pH of 7.5. The variation of NH3 fluxes simulated in this way showed a good agreement with those from the simulations with the original approach, accounting for a dynamically changing soil pH. These results suggest a way for model simplification when GAG_field is applied later at regional scale.

Lateral hydraulic performance of subsurface drip irrigation based on spatial variability of soil: Simulation

Soil physical properties (initial water content, bulk density, and mass fractal dimension) have a major influence on subsurface drip capillary water systems. The spatial distribution of lateral hydraulic performance varies along with the spatial variability of soil and is thus highly complex. In this paper, initial soil water content, soil bulk density, and mass fractal dimension generated according to a Gaussian distribution were used as input variables to a nonlinear lateral hydraulic mathematical model of subsurface drip irrigation. The numerical simulation indicated the following. 1) The greater the initial moisture content, soil bulk density and mass fractal dimension, the smaller was the lateral distribution of emitter discharge, and the smaller was the deviation rate of lateral flow. 2) The greater the standard deviation of initial moisture content, soil bulk density, and mass fractal dimension, the higher was the deviation rate of lateral flow, and the more laterally dispersed was the emitter discharge. 3) The greater the initial moisture content, soil bulk density, mass fractal dimension, and inner lateral diameter, the greater was the uniformity of lateral flow; The higher the inlet lateral pressure and emitter spacing, the lower was the uniformity of lateral flow; The uniformity of lateral flow increased with slope in the range −0.0003 to 1 and decreased with the increase of slope in the range 0–0.0005. The improved lateral hydraulic model by taking into account the influence of soil physical properties on subsurface drip capillary water systems, which permits identification of the critical points of the irrigation lateral

Wetting-drying cycles influence on soil respiration in two Mediterranean ecosystems

This study assesses which factors are involved in the soil respiration (Sr) response to wetting-drying cycles in two Mediterranean ecosystems. We analysed Sr, mineral nitrogen, ion-exchange resin mineral nitrogen, and phosphate levels at weekly intervals over one year in two Mediterranean ecosystems with contrasting characteristics: a pine forest with high levels of organic matter and nutrients and a shrubland with low carbon and nutrients availability. Higher Sr was detected in the pine forest (0.12–0.76 g CO2 m−2 hour−1) than in the shrubland (0.04–0.67 g CO2 m−2 hour−1). For both sites, Sr increased during wet periods and decreased during dry periods. Compared with Sr in the pine forest, the trend observed for resin mineral nitrogen was the opposite. No pattern was observed for resin mineral nitrogen at the shrubland site, or for mineral nitrogen or phosphate at either site. The initial water status of the wetting-drying cycles determined the Sr response, whereas the length of the drought period before the rewetting event had no effect. The impact of the initial soil water content on Sr played a crucial role when the wetting-drying events occur in a dry soil, having a secondary role in wet soils. Finally, soil water status drove Sr during the growing season in both ecosystems; however, soil temperature had no effect on CO2 efflux. In a changing world with projections of intensifying wetting-drying events, our results highlight the influence of soil water status on respiration rates, especially when these events occur in a dry soil.