With only 2.5% of the water on Earth available as fresh water, the security of its supply to feed a growing poplation will become increasingly uncertain. Global institutes try to find means to improve the distribution and security of water and food. Agriculture uses 70% of available fresh water which makes it by far the largest consumer of the limited amounts of fresh water. Water resources are finite and there is a need for new approaches to deal with increasingly complex water and food issues. Land and water management can contribute significantly to a sustainable increase of food security when based on modelling and monitoring in the soil water and plant domain.
Field scale experiments are needed to test new theories for their correctness and added predictive value with the soil water balance as a central core to explain impacts and changes of crop growth. Contribution of upward vertical water flow to roots is an essential part of the water balance and an important driver for transpiration of crops. A physical approach to quantify this vertical water flow should therefore be compared with more simplified approaches and be quantified using field experimental data.
The focus of this thesis is on vertical water flows. To be able to produce sound water balances actual yields have to be modelled, because actual transpiration is directly related to actual dry matter production. This in turn requires the ability to simulate actual crop growth and the need to account for actual water and crop management.
Nowadays modelling is common practice to analyse experiments at different scales, ranging from field to global scale. The natural domains described in this thesis require extensive field tests and verifications at different scales.
This thesis contributes to a better understanding of soil-water-plant interactions and to more advanced modelling of process-oriented approaches. It intends to provide an answer to four research questions :
What is the role of the vertical water flows capillary rise and recirculated percolation water in the determination of crop yields?
How can we model drought, salinity and oxygen crop stress to predict actual yields?
Can we predict the impact of different stresses on actual grassland production in the Netherlands?
How do changes of groundwater levels and land use affect actual crop yields and groundwater recharge?
This thesis starts in chapter 2 with a description of the agro-hydrological modelling tool SWAP 4 as it was applied in this thesis. A historical overview of 40 years modelling history is given followed by a brief description of Richards-based equations for soil water flow and a description of dynamic crop growth modelling. Solute transport is described as it was used in chapter 5 to simulate capillary rise of saline groundwater. Soil organic matter, soil nitrogen and crop nitrogen are described because these options are applied in chapter 6 to simulate nitrogen flows in soybean and soils in Argentina. Additional options are summarized with references to publications with detailed descriptions of the underlying processes.
SWAP 4 integrates modelling tools for Richards equation-based soil hydrology (SWAP) and process-oriented crop growth (WOFOST). This allows new ways to analyse the interaction of the two domains which is shown in the cases that are described in the chapters 3 – 6 of this thesis.
Chapter 3 quantifies the contribution of upward flow as capillary rise and separately as recirculated water to crop yield and groundwater recharge. Therefore we performed impact analyses of various soil water flow regimes on grass, maize and potato yields in the Dutch delta. The impact is clearly present in situations with relatively shallow groundwater levels (85% of the Netherlands), where capillary rise is a well-known source of upward flow; but also in free-draining situations the impact of upward flow is considerable. In the latter case recirculated percolation water is the flow source. To make this impact explicit we implemented a synthetic modelling option that stops upward flow from reaching the root zone, without inhibiting percolation. Such a hypothetically moisture-stressed situation compared to a natural one in the presence of shallow groundwater shows mean yield reductions for grassland, maize and potatoes of respectively 26, 3 and 14 % or respectively about 3.7, 0.3 and 1.5 ton dry matter per hectare. About half of the withheld water behind these yield effects originates from recirculated percolation water as occurs in free-drainage conditions and the other half comes from increased upward capillary rise. Soil water and crop growth modelling should consider both capillary rise from groundwater and recirculation of percolation water as this improves the accuracy of yield simulations. This also improves the accuracy of the simulated groundwater recharge: neglecting these processes causes overestimates of 17% for grassland and 46% for potatoes, or 63 and 34 mm yr-1, respectively.
Chapter 4 describes a general method to quantify the effects of hydrological measures on agricultural production in the Netherlands. The method is based on the hydrological simulation model SWAP and the crop growth model WOFOST. SWAP simulates water transport in the unsaturated zone using meteorological data, boundary conditions (like groundwater level or drainage) and soil parameters. WOFOST simulates crop growth as a function of meteorological conditions and crop parameters. The method allows a source-partitioning of stress-causes and results are illustrated for grassland and maize. The approach is tested using field scale experiments. The combination of these process-based models is also used to derive a meta-model, i.e. a set of easily applicable simplified relations for assessing crop growth as a function of soil type and groundwater level. These relations are based on multiple model runs for at least 72 soil units and the possible groundwater regimes in the Netherlands.
Chapter 5 describes the impact of future increasing salinity of groundwater, drought and water excess on grass production in the Netherlands. Models were first tested using datasets from field experiments and then applied at regional scale where we quantified the impact of various groundwater salinity levels on grass growth and production using a climate set of historical weather data. The results show that salinity effects on grass production are limited because the excess rainfall will infiltrate the soil and reduce salt water seepage. In a next step we used future weather data for the year 2050, derived from three Global Circulation Models and two CO2 emission scenarios. Salt stress mainly occurred when irrigation was applied with saline water. The increased CO2 concentration in combination with the limited drought stress resulted in increasing simulated actual and potential yields. Overall conclusion for grassland in the Netherlands is: drought stress is stronger than stress caused by water excess which on its turn is stronger than salinity stress. Future water demand for irrigation may increase by 11 – 19 % and results in water scarcity if water supply is insufficient.
Chapter 6 studies the changes of groundwater, climate and land use in the Pampas of Argentina. These changes offer opportunities and threats. Lowering groundwater without irrigation causes drought and successive crop and yield damage. Rising groundwater may alleviate drought as capillary rise supports root water uptake and improves crop growth. However rising groundwater may also limit soil water storage, cause flooding in metropolitan areas and have a negative impact on crop yields. Changing land use from continuous soy bean into crop rotations or natural vegetation may decrease groundwater recharge and thus decrease groundwater levels. However in case of crop rotation leaching of nutrients like nitrate may increase. We quantified these impacts using integrated dynamic crop growth and soil hydrology modelling. The models were tested at field scale using local dataset from Argentina. We applied distributed modelling at regional scale to evaluate impacts on groundwater recharge and crop yields using long term weather data. The experiments showed that threats come from continuous monotone land use with low evapotranspiration. Opportunities are created when a proper balance is found between supply and demand of soil water using a larger differentiation of land use. Increasing the areas of land use types with higher evapotranspiration, like permanent grassland and trees, will contribute to a more stable hydrology.
In the final chapter 7 answers to the 4 research questions are given:
What is the role of the vertical water flows?
In regions with groundwater levels less than two meters below the soil surface, the impact of capillary rise and recirculated percolation water is important. Both contribute equally to the upward water flow to the root zone as shown in chapter 3. In 85% of the Netherlands groundwater is within 2 meters depth, whereas in regions in the south and east of the Netherlands groundwater levels are deeper and capillary rise does not occur, but the process of recirculated percolation water occurs and should be taken into account. Neglecting the impact of capillary rise and recirculated percolation water implies neglecting production of dry matter and results in lower yields. For grassland, maize and potatoes we simulated yields reductions of 26, 3 and 14 % corresponding to 3.7, 0.3 and 1.5 ton dry matter per ha.
How to model stress to predict actual yields?
The impact of drought, salinity and oxygen on agricultural crop yields can be accurately quantified as demonstrated by an extensive comparison with field scale experiments. In order to achieve this we developed a new method that links dry, saline and wet conditions to an existing crop growth module. This new method can be used by farmers, regional governments, water boards and others to assess crop yield reductions due to groundwater changes.
How do different stresses affect actual grassland production?
In non-irrigated areas, which applies to about 82% of the Netherlands, droughts have a much larger impact on grass production than the excess of water and salinity. In chapter 5 we have modelled both current and future climate scenarios and generally found that increasing CO2 concentrations and temperatures stimulate grass growth and yields as long as water (rainfall) is sufficient. Increasing salt concentrations and water excess have a limited effect on grass production. The considered future climate scenarios generally show increasing potential and actual yields as well as an increased demand for irrigation water of 11-19%. Irrigation will compensate water deficit and contribute to increasing yields but only in those parts of the Netherlands where enough irrigation water of sufficient quality is available.
How do groundwater levels and land use affect actual crop yields and groundwater recharge?
Increasing groundwater levels generally benefit crop yields because capillary rise decreases drought stress in the root zone. This was analysed for soybean growth in the pampas of Argentina. The long-term agricultural land use of no-tillage growth of soybean has a strong impact on groundwater levels because of its relatively low evapotranspiration. This results in reduced groundwater recharge and increasing groundwater levels. Changing land use from soybean into crop rotations has a limited effect on the reduction of groundwater recharge and groundwater levels. In the case of crop rotation leaching of nutrients like nitrate may increase. When groundwater levels get close to the soil surface flooding will occur. More variation and especially more continuous types of land use such as grassland and forest are therefore recommended as one of the options to lower groundwater levels and thereby increase storage capacity of the soil and reduce flooding risk.
Complex modelling is often necessary and provides valuable insight into the processes underlying the interaction between soil hydrology and crop growth. With this thesis it is shown that a Richards equation-based simulation of water flow to root zone and crops contributes to improved estimates of crop yield. Neglecting these processes will overestimate the groundwater recharge. As a result of this, the availability of drinking water and the risk of flooding may be overestimated.