Monthly disaggregation of annual irrigation water demand in the southern Murray Darling Basin

Water demand forecasting plays a crucial role in the efficient planning and management of water distribution systems, particularly in regions facing complex climatic and irrigation dynamics. Fluctuations in water demand occur across both seasonal and sub-seasonal timeframes, driven by diverse factors including weather variations and irrigation management choices. Traditionally, irrigation water demands have been forecasted separately for these two temporal scales, using only subsets of factors most relevant to each specific temporal scale. Sub-seasonal water demands are, however, influenced by annual decisions. This is particularly true in large, complex water systems with extensive water trade and diverse climate conditions. For such systems, a sub-seasonal scale forecast with consideration of annual influencing factors could add significant value to operational management. This paper presents an empirical approach to disaggregate the annual allocation water use for irrigation to a monthly time scale for seven inter-connected regions of the southern Murray-Darling Basin (sMDB), Australia. First, an extensive literature review was conducted to identify the crop coefficient (Kc) values for the crops grown in the sMDB throughout their growth cycles. Following this, a set of monthly Kc values were adopted for nine irrigation activities. Subsequently, the annual allocation water use was disaggregated into monthly volumes using within year proportion of crop water requirement, calculated from reference evapotranspiration and crop coefficients for the seven regions in the sMDB. Finally, the results of the disaggregation approach were compared against the diversion data matched to each region. The disaggregated allocation water use aligns well with the monthly diversion volume. Based on this approach, a monthly water demand forecast model accounting for annual and monthly influencing factors could be developed.