Irrigation and Ecosystem Services: development of an irrigation model for the LUCI ecosystem services framework
Poor water quality is currently a major environmental issue worldwide and in New Zealand, where reactive Nitrogen (N) and Phosphorous (P) lost from agricultural fields are significant drivers of water quality degradation in rural catchments. Irrigation application to crops is essential to agricultural production however irrigation inputs can increase N and P losses to waterways via drainage and/or overland flow directly and as a result of reduced soil capacity to buffer rainfall events. Indirect nutrient losses are also increased following irrigation implementation due to amplified farming intensity. Furthermore, irrigation applications represent the world’s greatest consumptive use of water. Improving irrigation efficiency with regard to water use represents a synergistic opportunity for the improvement of a number of different ecosystem services including water quality, water supply, and food production. Spatially explicit modelling of irrigation is needed to determine inefficiencies in water delivery and target these inefficiencies for management or mitigation at sub-field scales. A complimentary need exists for irrigation modelling within ecosystem service decision support tools so that nutrient and water movement can be accurately quantified in irrigated environments. This thesis describes the development and implementation of SLIM – the Spatially-explicit LUCI Irrigation Model. SLIM adapts existing lumped hydrological and irrigation modelling techniques and practices to a fully distributed, spatially explicit framework, so that sub-field variations in water flows resulting from variable soil properties are accounted for. SLIM is generally applicable across New Zealand, using readily available national scale datasets and literature derived parameters. SLIM is capable of predicting irrigation depth and timing based on common management strategies and irrigation system characteristics, or can replicate irrigation applications where information is available. Outputs from SLIM are designed to assist irrigation management decisions at the field level, and to inform the hydrology component of the Land Utilisation and Capability Indicator (LUCI) ecosystem service assessment framework. Standalone SLIM outputs include time-series files, water balance plots, and raster maps describing the efficiency and efficacy of the modelled irrigation system. SLIM has been applied in three different agroecosystems in New Zealand under surface, micro, and spray irrigation systems, each characterised by different levels of data availability. Results show that SLIM is able to accurately predict the timing of irrigation applications and provide usable information to inform irrigation application decisions. SLIM outputs emphasise the importance of soil variability with regard to water loss and risk of nutrient leaching. Opportunity exists for irrigation water use efficiency to be improved through targeted management at sub-field scales in New Zealand farming systems.