Better Prediction of Groundwater Recharge From Rice Growing

Abstract

A range of restrictions are associated with rice growing – land suitability, target rice water use, buffer areas and hydraulic loading limits. The Land and Water Management Planning process undertaken in southern New South Wales Irrigation Areas and Districts clearly identified the need to improve the rice land suitability criteria. Improvements to rice soil suitability criteria (including soil sodicity) are required due to the continuing restrictions to irrigation water availability, along with the need to limit recharge to regional groundwater systems. These criteria will assist irrigators maximize rice water use efficiency by allowing selection of the most suitable soils for rice growing. The Problem- Although the EM31 instrument, GPS and computer mapping technology have enabled significant improvements in the assessment of rice land to reduce groundwater accessions, modifications to the soil assessment process are needed. Cumulative infiltration during rice growing is known to vary significantly over four broad soil categories: self mulching clay soils, non self mulching clay soils, near levee soils and transitional red brown earths, there are also large differences in the level of infiltration within these soil categories (Van der Lelij and Talsma, 1978). Localised sites, which have high infiltration rates and thus allow high levels of groundwater recharge, may exist within rice fields and their delineation and exclusion or modification is an important aspect of rice land management in the southern Australian rice industry. Many of the soils in southern NSW are sodic. In sodic soils both swelling and dispersion of the soil occur. Swelling and dispersion reduce soil infiltration, permeability and ultimately deep drainage/ groundwater recharge. The Objective- The overall project aim was to improve rice land soil suitability identification and assessment approaches. The primary objective of this work was to investigate ways of refining the electromagnetic (EM) technology approach to include soil chemical characteristics specifically soil sodicity or exchangeable sodium percentage, in the rice land assessment process. Additional objectives were to: identify if the EM31 horizontal mode or EM38 provide better definition of the suitable rice land than the currently used EM31 vertical mode; and identify if land with ECa < 50 mS/m (EM31v) can be classified as unsuitable for rice without further determination of soil properties. The final objective was to promote adoption of the findings to date, particularly among irrigation company staff, DLWC regulatory staff and EM service providers to industry.