Impact of climatic trends on the water demand and irrigation planning for annual crops in the Sao Francisco River basin, Brazil

This study evaluated the spatial distribution of precipitation (P) and Peman-Monteith reference ET (ETo) in Brazil's S & atilde;o Francisco River basin (1961-2022). The analysis included Mann-Kendall trend tests and future IPCC projections on P and ETo to assess the need for supplemental irrigation (SI) of maize crop over twelve hypothetical growing seasons. The average annual P in the basin was 1300 mm year(-1), decreasing from the Upper to the Lower section. Annual ETo averaged 1670 mm year(-1), with higher depths in the Middle and Lower-middle sections. Trends for both P and ETo were significant(p < 0.05) in 58% of the basin. The overall trend magnitude in P was positive (average of about 0.5 mm year(-2)). Trend was decreasing in 56% of the sites (3.2 mm year(-2)) and increasing in 44% of them (5.3 mm year(-2)). For ETo, the overall trend was also positive (average of 3.0 mm year(-2)), with increasing trend in 79% of the sites (4.6 mm year(-2)) and a decreasing trend of 3.8 mm year(-2) in 21% of the area. The need of SI varied both over the year with peaks in the May-August period and over the basin with higher depths in the Middle and Lower-middle sections. Large SI values (>800 mm) were associated with the dry season and areas with persistent trends in decreasing P and increasing ETo. Future IPCC projections indicate increased SI depths in most of the basin by 2081-2100, with the Middle and Lower-middle sections facing potentially unsustainable level for crop production.