Minimising the environmental impacts of irrigation by good scheduling

Irrigation constitutes a major user of water resources at a time, and in places, where resources are at their lowest. Also, by maintaining the soil in a wetter state, it increases the annual drainage, which can affect the leaching of nitrates into the groundwater. Irrigation scheduling involves, firstly, deciding the most appropriate irrigation plan (i.e. what soil water deficit to allow and how much to apply at that deficit) and secondly, deciding what is the soil water deficit on any particular day. Good scheduling will aim to meet the goals of irrigation (optimise production / quality / aesthetics) whilst minimising the water used and other adverse environmental impacts. The main techniques for scheduling irrigation in the UK are direct measurement of soil water content and water balance modelling. ‘Bad’ irrigation scheduling can result from an inappropriate irrigation plan, inaccurate soil water measurement, errors in water balance modelling or uncertainty over the actual amount of water applied at each irrigation. Either will result in under- or over-watering. A case study for potatoes grown in a medium textured soil in Silsoe (Beds) examined the impact of poor scheduling on average annual irrigation water requirement and the risk of nitrate leaching. The water requirement is very sensitive to errors in estimating evapotranspiration or the field-capacity water content of the soil, but less so to errors in the amount of water applied. Increasing the trigger soil water deficit can also minimise the water requirement. Over-irrigation, due to a poor irrigation plan can increase the summer drainage and risk of nitrate leaching, however, over-watering due to inaccurate scheduling predominantly results in increased winter drainage which may have a positive impact on nitrate concentrations and winter recha