Soil chemical aspects of water management: modeling topsoil water, salt and sodicity dynamics

In large areas worldwide, both in semi-arid and humid climates, the availability of good quality water for primary production in agriculture or natural vegetations is limited. Whereas soil and groundwater pollution are nowadays general problems, much larger areas have to deal with the hazards of soil and groundwater salinity. For soil types that are susceptible to physical degradation in view of their swelling and shrinking behaviour, also the salinity associated hazard of soil sodicity needs to be anticipated. Sodicity related soil physical degradation may be quite irreversible and is a major candidate for preventive rather than curative measures. The process of salt displacement has received much attention, and spatial variability has been given explicit attention by Dagan and Bresler (1979). Temporal variability of the boundary conditions has been investigated only recently, when both primary and secondary salinity were studied with so-called minimalist ecohydrological modelling (Suweis et al., 2010, Shah et al., 2011, Vervoort and Van der Zee, 2012). This work emphasized the atmospheric forcing of erratic rainfall on water and salt dynamics in the rootzone layer, either envisioned as a Poisson process, or with seasonal periodicity. Taking such erratic rainfall as well as spatial variability of the depth of the groundwater level into account, an impression can be obtained of the importance of different sources of variability. For our parameterization, we find that both spatial and temporal variability were responsible for the variation in root zone salinity, C, but that root zone sodicity, expressed with the Exchangeable Sodium Percentage, ESP, depends predominantly on spatial variability. The difference is understandable from the buffering mechanisms involved with C and ESP

Modeling the effects of saline groundwater and irrigation water on root zone salinity and sodicity dynamics in agro-ecosystems

Recent trends and future projections suggest that the need to produce more food and fibre for the world’ s expanding population will lead to an increase in the use of marginal-quality water and land resources (Bouwer, 2000; Gupta and Abrol, 2000; Wild, 2003). This is particularly relevant to less-developed, arid and semi-arid countries, in which problems of soil and water quality degradation are common (Qadir and Oster, 2004). The aim, therefore, should be to increase yield per unit of land rather than the area cultivated. More efforts are needed to improve productivity as more lands are becoming degraded. It is estimated that about 15% of the total land area of the world has been degraded by soil erosion and physical and chemical degradation, including soil salinization (Wild, 2003). The main sources of soil salinity and sodicity development are groundwater and irrigation water. In discharge areas of the landscape, water exits from groundwater to the soil surface bringing the salts dissolved in it. The driving force for upward movement of water and salts is evaporation from the soil plus plant transpiration. Salt accumulation is high when the water table depth is less than a threshold. However, this threshold depth may vary depending on soil hydraulic properties and climatic conditions. Groundwater associated salinity and sodicity affects around 350 X 104km2in the world (Szabolcs, 1989). In this thesis, the focus is to quantify and understand the salinity and sodicity dynamics, and the feedback on dynamics in groundwater dependent agro-ecosystems. First we have considered the impact of salt coming from groundwater on capillary fluxes and on the root zone water and salt dynamics. Groundwater can be a source of both water and salts in semi-arid areas, and therefore capillary pressure induced upward water flow may cause root zone salinization. To identify which conditions result in hazardous salt concentrations in the root zone, we combined the mass balance equations for salt and water, further assuming a Poisson-distributed daily rainfall and brackish groundwater quality. For the water fluxes (leaching, capillary upflow, and evapotranspiration), we account for osmotic effects of the dissolved salt mass using Van‘t Hoff’s law. Root zone salinity depends on salt transport via capillary flux and on evapotranspiration, which concentrates salt in the root zone. Both a wet climate and shallow groundwater lead to wetter root zone conditions, which in combination with periodic rainfall enhances salt removal by leaching. For wet climates, root zone salinity (concentrations) increases as groundwater is more shallow (larger groundwater influence). For dry climates, salinity increases as groundwater is deeper due to a drier root zone and less leaching. For intermediate climates, opposing effects can push the salt balance in either way. Root zone salinity increases almost linearly with groundwater salinity. With a simple analytical approximation, maximum concentrations can be related with the mean capillary flow rate, leaching rate, water saturation and groundwater salinity, for different soils, climates and groundwater depths. A Soil sodicity (quantified by ESP) model based on the soil salinity model (as discussed above) has been developed. For sodicity calculations, we have used the Gapon equation favored in salinity research. The simulation results show that soil salinity and sodicity development in groundwater driven agro-ecosystems play a major role in soil structure degradation. To identify which conditions can make soil sodic, we have modeled the coupled water, salt, and cation balances. The root zone salinity Cand sodicity ESPgradually change to their long term average values. These long term average values are independent of the cation exchange capacity CEC. The rate of change depends inversely on the size of the root zone reservoir, i.e., on root zone thickness for C, and additionally on CEC, for ESP.Soil type can have a large effect on both the rate of approach of the long term steady state salinity and sodicity, and on the long term levels, as it affects the incoming and out-going water and chemical fluxes. Considering two possible sources of salts, i.e., groundwater and irrigation water (here represented by rainfall), the long term salt concentration Cof the root zone corresponds well with a flux weighted average of infiltrating and upflowing salt mass divided by the average water drainage. In full analogy, the long term ESPcan be approximated very well for different groundwater depths and climates. A more refined analytical approximation, based on the analytical solution of the water balance of Vervoort and Van der Zee(2008), leads to a quite good approximation of long term salinity and sodicity, for different soils, groundwater depths, and climates. Modeling is an efficient tool to investigate water and solute movement in groundwater driven agro-ecosystems. However, in most available models (SWAP,MODFLOW/MT3D) continuing degradation of soil hydraulic properties as a result of rising Na+concentrations is ignored. Disregarding the soil hydraulic degradation due to sodicity level in some cases makes modeling water and solute movement within the soil profile questionable. We have translated the effects of soil salinity and sodicity into reduction in saturated hydraulic conductivity to quantify the feedback effects of reduction in saturated hydraulic conductivity on root zone fluxes, salinity, and sodicity under different groundwater depths and climates of Oenpelli and Tennant Creek Airport located in the North Territory of Australia. The reduction in saturated hydraulic conductivity due to salinity and sodicity (Ks(C,ESP))has been calculated by using the procedure developed by McNeal(1968). The significant feedback effects of Ks(C,ESP) on salt concentration and soil ESPdepend on many important parameters like groundwater depth, leaf area index, weather seasonality and non-seasonality, and soil type. Out of these important parameters, weather seasonality is the main driver that can develop significant feedback effects of Ks(C,ESP) onsalt concentration and soil ESP. Furthermore, Ks(C,ESP) although decreasing the capillary flux, leaching flux, and evapotranspiration, it increases the magnitude of runoff. Also when Ks(C,ESP)affects both capillary and leaching flux under seasonal rainfall, the feedback effects are significant compared to the partial feedback (Ks(C,ESP)affects only leaching flux, but not capillary flux). In the second theme of this thesis, we have focused on optimizing irrigation water between two farms under water scarcity and salinity regimes. In arid and semi-arid regions, irrigation water is scarce and often saline. To reduce negative effects on crop yields, the irrigated amounts must include water for leaching and therefore exceed evapotranspiration. The leachate (drainage) water returns to water sources such as rivers or groundwater aquifers and increases their level of salinity and the leaching requirement for irrigation water of any sequential user. We develop a sequential (upstream-downstream) model of irrigation that predicts crop yields and water consumption and tracks the water flow and level of salinity along a river dependent on irrigation management decisions. The model incorporates an agro-physical model of plant response to environmental conditions including feedbacks. For a system with limited water resources, the model examines the impacts of water scarcity, salinity and inefficient application on yields for specific crop, soil, and climate conditions. As a general pattern we find that, as salinity level and technical inefficiency increase, the system benefits when upstream farms use less water than is available to them, to provide downstream farms with more and better quality water. We compute the marginal value of water, i.e. the price water that would command on a market, for different levels of water scarcity, salinity and levels of water loss. In summary this thesis aims to understand theoretically how soil salinity and sodicity develop under different climates, groundwater depths, soil types, root zone thicknesses, and different groundwater salinities. The developed salinity sodicity model can be applied in potential salt affected areas to predict the long term salinity, sodicity trends. Furthermore, quantification of feedback effects of reduction in saturated hydraulic conductivity (Ks(C,ESP)) on root zone fluxes, salinity, and sodicity guide us towards better management of soil, vegetation, and irrigation/groundwater. </p

Root zone salinity and sodicity under seasonal rainfall due to feedback of decreasing hydraulic conductivity

Soil sodicity, where the soil cation exchange complex is occupied for a significant fraction by Na+, may lead to vulnerability to soil structure deterioration. With a root zone flow and salt transport model, we modeled the feedback effects of salt concentration (C) and exchangeable sodium percentage (ESP) on saturated hydraulic conductivity Ks(C, ESP) for different groundwater depths and climates, using the functional approach of McNeal (1968). We assume that a decrease of Ks is practically irreversible at a time scale of decades. Representing climate with a Poisson rainfall process, the feedback hardly affects salt and sodium accumulation compared with the case that feedback is ignored. However, if salinity decreases, the much more buffered ESP stays at elevated values, while Ks decreases. This situation may develop if rainfall has a seasonal pattern where drought periods with accumulation of salts in the root zone alternate with wet rainfall periods in which salts are leached. Feedback that affects both drainage/leaching and capillary upward flow from groundwater, or only drainage, leads to opposing effects. If both fluxes are affected by sodicity-induced degradation, this leads to reduced salinity (C) and sodicity (ESP), which suggests that the system dynamics and feedback oppose further degradation. Experiences in the field point in the same direction

Root zone salinity and sodicity under seasonal rainfall due to feedback of decreasing hydraulic conductivity

Soil sodicity, where the soil cation exchange complex is occupied for a significant fraction by Na+, may lead to vulnerability to soil structure deterioration. With a root zone flow and salt transport model, we modeled the feedback effects of salt concentration (C) and exchangeable sodium percentage (ESP) on saturated hydraulic conductivity Ks(C, ESP) for different groundwater depths and climates, using the functional approach of McNeal (1968). We assume that a decrease of Ks is practically irreversible at a time scale of decades. Representing climate with a Poisson rainfall process, the feedback hardly affects salt and sodium accumulation compared with the case that feedback is ignored. However, if salinity decreases, the much more buffered ESP stays at elevated values, while Ks decreases. This situation may develop if rainfall has a seasonal pattern where drought periods with accumulation of salts in the root zone alternate with wet rainfall periods in which salts are leached. Feedback that affects both drainage/leaching and capillary upward flow from groundwater, or only drainage, leads to opposing effects. If both fluxes are affected by sodicity-induced degradation, this leads to reduced salinity (C) and sodicity (ESP), which suggests that the system dynamics and feedback oppose further degradation. Experiences in the field point in the same direction

A coupled agronomic-economic model to consider allocation of brackish irrigation water

[1] In arid and semiarid regions, irrigation water is scarce and often contains high concentrations of salts. To reduce negative effects on crop yields, the irrigated amounts must include water for leaching and therefore exceed evapotranspiration. The leachate (drainage) water returns to water sources such as rivers or groundwater aquifers and increases their level of salinity and the leaching requirement for irrigation water of any sequential user. We develop a conceptual sequential (upstream-downstream) model of irrigation that predicts crop yields and water consumption and tracks the water flow and level of salinity along a river dependent on irrigation management decisions. The model incorporates an agro-physical model of plant response to environmental conditions including feedbacks. For a system with limited water resources, the model examines the impacts of water scarcity, salinity and technically inefficient application on yields for specific crop, soil, and climate conditions. Moving beyond the formulation of a conceptual frame, we apply the model to the irrigation of Capsicum annum on Arava Sandy Loam soil. We show for this case how water application could be distributed between upstream and downstream plots or farms. We identify those situations where it is beneficial to trade water from upstream to downstream farms (assuming that the upstream farm holds the water rights). We find that water trade will improve efficiency except when loss levels are low. We compute the marginal value of water, i.e., the price water would command on a market, for different levels of water scarcity, salinity and levels of water los

Soil sodicity as a result of periodical drought

Soil sodicity development is a process that depends nonlinearly on both salt concentration and composition of soil water. In particular in hot climates, soil water composition is subject to temporal variation due to dry–wet cycles. To investigate the effect of such cycles on soil salinity and sodicity, a simple root zone model is developed that accounts for annual salt accumulation and leaching periods. Cation exchange is simplified to considering only Ca/Na exchange, using the Gapon exchange equation. The resulting salt and Ca/Na-balances are solved for a series of dry/wet cycles with a standard numerical approach. Due to the nonlinearities in the Gapon equation, the fluctuations of soil salinity that may be induced, e.g. by fluctuating soil water content, affect sodicity development. Even for the case that salinity is in a periodic steady state, where salt concentrations do not increase on the long term, sodicity may still grow as a function of time from year to year. For the longer term, sodicity, as quantified by Exchangeable Sodium Percentage (ESP), approaches a maximum value that depends on drought and inflowing water quality, but not on soil cation exchange capacity. Analytical approaches for the salinity and sodicity developing under such fluctuating regimes appear to be in good agreement with numerical approximations and are very useful for checking numerical results and anticipating changes in practical situation

Stochastic modeling of salt accumulation in the root zone due to capillary flux from brackish groundwater

Groundwater can be a source of both water and salts in semiarid areas, and therefore, capillary pressure–induced upward water flow may cause root zone salinization. To identify which conditions result in hazardous salt concentrations in the root zone, we combined the mass balance equations for salt and water, further assuming a Poisson-distributed daily rainfall and brackish groundwater quality. For the water fluxes (leaching, capillary upflow, and evapotranspiration), we account for osmotic effects of the dissolved salt mass using Van‘t Hoff's law. Root zone salinity depends on salt transport via capillary flux and on evapotranspiration, which concentrates salt in the root zone. Both a wet climate and shallow groundwater lead to wetter root zone conditions, which in combination with periodic rainfall enhances salt removal by leaching. For wet climates, root zone salinity (concentrations) increases as groundwater is more shallow (larger groundwater influence). For dry climates, salinity increases as groundwater is deeper because of a drier root zone and less leaching. For intermediate climates, opposing effects can push the salt balance either way. Root zone salinity increases almost linearly with groundwater salinity. With a simple analytical approximation, maximum concentrations can be related to the mean capillary flow rate, leaching rate, water saturation, and groundwater salinity for different soils, climates, and groundwater depths

Soil sodicity as a result of periodical drought

Soil sodicity development is a process that depends nonlinearly on both salt concentration and composition of soil water. In particular in hot climates, soil water composition is subject to temporal variation due to dry-wet cycles. To investigate the effect of such cycles on soil salinity and sodicity, a simple root zone model is developed that accounts for annual salt accumulation and leaching periods. Cation exchange is simplified to considering only Ca/Na exchange, using the Gapon exchange equation. The resulting salt and Ca/Na-balances are solved for a series of dry/wet cycles with a standard numerical approach. Due to the nonlinearities in the Gapon equation, the fluctuations of soil salinity that may be induced, e.g. by fluctuating soil water content, affect sodicity development. Even for the case that salinity is in a periodic steady state, where salt concentrations do not increase on the long term, sodicity may still grow as a function of time from year to year. For the longer term, sodicity, as quantified by Exchangeable Sodium Percentage (ESP), approaches a maximum value that depends on drought and inflowing water quality, but not on soil cation exchange capacity. Analytical approaches for the salinity and sodicity developing under such fluctuating regimes appear to be in good agreement with numerical approximations and are very useful for checking numerical results and anticipating changes in practical situations.Dry-wet cycles Root zone model Salinity Sodicity Water management Salt accumulation Leaching Ecohydrology

Soil sodicity as a result of periodical drought

Soil sodicity development is a process that depends nonlinearly on both salt concentration and composition of soil water. In particular in hot climates, soil water composition is subject to temporal variation due to dry–wet cycles. To investigate the effect of such cycles on soil salinity and sodicity, a simple root zone model is developed that accounts for annual salt accumulation and leaching periods. Cation exchange is simplified to considering only Ca/Na exchange, using the Gapon exchange equation. The resulting salt and Ca/Na-balances are solved for a series of dry/wet cycles with a standard numerical approach. Due to the nonlinearities in the Gapon equation, the fluctuations of soil salinity that may be induced, e.g. by fluctuating soil water content, affect sodicity development. Even for the case that salinity is in a periodic steady state, where salt concentrations do not increase on the long term, sodicity may still grow as a function of time from year to year. For the longer term, sodicity, as quantified by Exchangeable Sodium Percentage (ESP), approaches a maximum value that depends on drought and inflowing water quality, but not on soil cation exchange capacity. Analytical approaches for the salinity and sodicity developing under such fluctuating regimes appear to be in good agreement with numerical approximations and are very useful for checking numerical results and anticipating changes in practical situation