Trends in streamflow in the Ibicui river basin -influence of rice crop irrigation

摘要

The Ibicui basin, located in the south of Brazil, is close to 50,000 km2 in drainage area. The basin has big problems with water deficits, attributed to the indiscriminate use of water to irrigate rice. The objective of this study is a statistical analysis of water flow data in the Ibicui basin to verify if there are significant trends in water availability related to the withdrawal of water for rice crop irrigation. We used data from 11 fluviometric stations for 1970 to 2011, corresponding to the period of major growth in rice cultivation. Records of daily flow data were normalized, then for each month, the flow at durations between 50% to 99% were calculated. Trends in these series were evaluated using the Mann-Kendall test. The results showed that there are trends of increasing water flow for 8 of the 11 stations, and in 6 of those 8 stations the increasing trend was statistically significant. Just 3 stations had negative trends and these were in sub-basins with higher percentage area in rice. Analyzing the trends for several flow durations, it was observed that there was a reduction of the trends with duration. Also, in a river with sequential stations, the significance of trends as reflected by the Mann-Kendall Zs decreased with irrigated area. We conclude that for the Ibicui Basin analysis of trends in the flow data does not clearly reflect the effect of water withdrawals for irrigation of rice.