Modelling the long-term impact of modernized irrigation systems on soil water and salt balances, and crop water productivity in semi-arid areas under current and potential climate change conditions : integration of agrohydrological model, geographical information system, remote sensing, and climate change model : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University, Palmerston North, New Zealand
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2022
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Massey University
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Abstract
Irrigated agriculture plays a key role in ensuring food security and rural livelihoods across semi-arid and arid regions, like in the Indus basin of Pakistan. However, the Indus basin irrigation system of Pakistan is facing serious threats of low crop yields and increasing water scarcity, waterlogging, soil salinity, and overexploitation of groundwater. Considering the irrigation water-management issues, water managers and policymakers in Pakistan are looking into the modernization of the irrigation practices by introducing sprinkler and drip irrigation systems with the intent to save water and enhance crop water productivity. However, such intervention if adopted at a larger scale could seriously affect regional soil water and salt balances, solute leaching, and recharge to groundwaters in semi-arid and arid regions. Therefore, a robust assessment of the long-term potential impacts of modernised irrigation systems, particularly under the potential climate change scenarios, is essential for improving productivity and sustainable irrigated agriculture in semi-arid and arid regions.
Field experiments are practically difficult to quantify the long-term impacts of modernised irrigation practices on soil water and salt balances and crop growths, especially under projected climate change conditions. This thesis developed a modelling framework using local field experiments, and geographical and remote sensing information, combined with a spatially distributed agrohydrological model and climate change projections to analyse the potential impacts of different irrigation application scenarios at the field and canal command scales. This methodology is applied to evaluate the potential impacts of current and proposed modernized irrigation systems on soil water and salt balances, soil salinity build-up, percolation to groundwaters, crop yield and crop water productivity of irrigated crops under long-term contemporary climate (1987-2017) and potential climate change (2070-2099) scenarios. The main irrigated crops of wheat, rice, and cotton were studied in the Hakra branch canal command as a case study.
The Hakra branch canal (HBC) command, located in the Indus basin irrigation system of Pakistan, covers 0.21 million ha and is characterised by the typical problems of canal water scarcity, poor groundwater quality, waterlogging and soil salinity, and less-than-optimal crop production. The information collected from local field-scale experiments during the years 2016-2017, GIS, remote-sensing techniques and global climate models are integrated to parametrise, calibrate, and validate the agrohydrological Soil-Water-Atmosphere-Plant (SWAP) model application at both field- and canal command- scales. The SWAP model simulated soil water and salt balances, percolation to groundwaters, and water- and salt-limited crop yields and crop water productivity values of main irrigated crops of wheat, rice, and cotton from field- to canal command- scales in the study area.
The modelling assessment of current irrigation practices revealed significant variation in canal water supplies and over-exploitation of groundwater, resulting in high spatial variability in soil water percolation and salt build-up in the soil at the spatial scale of the head, middle and tail reaches of the canal command. The canal water-inflow is about 19% and 42% higher at the head reaches than at the middle and tail reaches, respectively. The significant seepage from the canal network and the cultivation of high water-consuming crops such as rice are the potential cause of waterlogging at the head reaches. Whereas limited canal inflow and use of poor-quality groundwater (> 3 dS m⁻¹) appear to be potential causes of soil salinity at the tail reaches of the HBC command. The detrimental effects of limited canal inflow and the use of marginal to poor groundwater causes considerable spatial variation in simulated water and salt-limited crop yields. The simulated water and salt-limited crop water productivity values are not only different for the different crops of wheat, rice and cotton, but also for the same crop across the study area.
The field- and canal-command scale modelling was applied to simulate and assess the potential impacts of the proposed modernized irrigation scenarios, such as
• sprinkler irrigation is defined as a high-efficiency irrigation system with leaching fraction (HEIS_LF) and without leaching fraction (HEIS_noLF), and
• precision surface irrigation system (PSIS) for cotton-wheat cultivation under contemporary climate (1987-2017) and potential climate change (2070-2099) scenarios RCP 2.6 (low emission) and RCP 8.5 (high emission or business-as-usual).
The long-term simulation results suggest a saving of about 40% in irrigation water under the HEIS_noLF scenario. However, this irrigation water-saving under the HEIS_noLF scenario resulted in the risk of an increase in soil salinity due to reduction in soil percolation and its associated salt build-up in the soil profile. Under the HEIS_noLF scenario for cotton-wheat cultivation, the soil salinity is simulated to increase from 2.6 to 8.0 dS m⁻¹ at the field-scale, and from 2 to >12 dS m⁻¹ at the canal command scale, affecting crop yields due to salt stress. The high salt build-up is simulated to reduce crop yields by 38% for cotton, and 48% for wheat under the contemporary climate (1987-2017) at the canal command scale. The soil salinity is simulated to get even worse in poor-quality groundwater areas, resulting in wheat failure of < 1 ton/ha with HEIS_noLF under the RCP 8.5 scenario of potential climate change (2070-2099) conditions.
The modelling analysis suggests a significant leaching fraction is required to maintain acceptable soil salt balance for successful crop production. This leaching fraction could be achieved by a pre-sowing irrigation of 60 mm depth at the start of the season, followed by an additional 10 mm depth with each irrigation interval using a high-efficiency irrigation application, simulated as HEIS_LF. The HEIS_LF scenario resulted in 50 to 65% higher average water- and salt-limited crop water productivity values (kg/m³ ET) of 0.5 for cotton, and 1.87 for wheat. This is compared to the HEIS_noLF scenario of 0.25 for cotton, and 0.65 for wheat under potential climate change (2070-2099) conditions. However, the PSIS irrigation scenario resulted in similarly favourable soil water and salt balances, water and salt-limited crop yields and crop water productivity values for the cotton - wheat cultivation. Under the PSIS irrigation scenario, the average water-and salt-limited crop water productivity values (kg/m³ ET) are simulated as 0.50 for cotton and 2.79 for wheat under the contemporary climate (1987-2017), and 0.50 for cotton and 1.92 for wheat in potential climate change (2070-2099) conditions. The modelling analysis simulated the average soil percolation rate as 10 to 20% higher, resulting in the leaching of 20 to 30% more salts from the soil profile under the PSIS scenario than the HEIS_LF under potential climate change conditions.
The key findings of this modelling assessment suggest that modernisation of irrigation systems as higher-efficiency (HEIS) irrigation applications, with no appropriate leaching fraction, would compromise salt build-up in the soil profile. This would potentially reduce crop yields and crop water productivity in the long-term, especially under potential climate change (2070-2099) conditions. There appears very limited scope for real irrigation water savings using a high-efficiency irrigation system for long-term sustainable crop production in areas making conjunctive use of limited canal water supplies and marginal- to poor-quality groundwaters. Hence, proposed initiatives for implementing high-efficiency irrigation systems should be carefully evaluated in terms of their long-term potential impacts on regional soil water and salt balances, crop yields and crop water productivity values in areas such as the Indus basin irrigation system in Pakistan, particularly under potential climate-change conditions.
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Figure 2.1 is reused under a Creative Commons Attribution 3.0 License. Figure 2.4 (=Latif, 2007 Fig 1) was removed for copyright reasons.
Keywords
Irrigation farming, Irrigation efficiency, Mathematical models, Irrigation, Environmental aspects, Pakistan, Punjab, high-efficiency irrigation system, precision surface irrigation system, soil water and salt balances, soil salinity, crop water productivity, SWAP model, climate change, Indus basin irrigation system of Pakistan