Trends in streamflow in the Ibicui River basin, Brazil: influence of rice crop irrigation.

摘要

The Ibicui River basin in Rio Grande do Sul, Brazil, has recurring problems of water deficits, commonly attributed to the indiscriminate use of water to irrigate rice. The objective of this study was a statistical analysis of water flow data in the Ibicui basin to verify if significant trends in water availability are related to the withdrawal of water for rice crop irrigation. We used data from 11 fluviometric stations for 1970 to 2011, which corresponds to the period of major expansion in rice cultivation in the basin. Daily flows were normalized, and for each month the flow rate was calculated for durations from 50% to 99%. Trends in these series were evaluated using the Mann-Kendall test. Results show that over the 40-year period of study there are trends of increasing water flow for eight of the 11 stations, and at six of those eight stations the increasing trend was statistically significant. In a river with sequential stations, differences between the stations indicated a decreasing trend for durations >80% as reflected by the Mann-Kendall Z_s. However, the primary analysis failed to show trends that might reflect increasing irrigation withdrawals. Thus, contrary to expectations, we conclude that, for the Ibicui basin, analysis of trends in the flow data does not clearly implicate water withdrawals for rice irrigation as the reason for water deficits.