Effect of Rice Intensification on the Quality and Quantity of Runoff Water

Rice cultivation under flood irrigation presents a high-water demand, associated not only with evapotranspiration requirements but also with losses due to percolation, lateral seepage, and surface runoff. The latter, in addition to affecting water-use efficiency, may constitute a potential pathway for nutrient exports to surface water bodies. In particular, nitrogen (N) and phosphorus (P) have been widely identified as the nutrients of greatest environmental concern due to their central role in eutrophication processes. At the regional scale, this issue is especially relevant in the Laguna Merín Binational Basin (LMB), the main rice-growing region of Uruguay, where elevated total phosphorus (TP) concentrations exceeding regulatory standards have been recorded at most monitoring sites. In this context, the general objective of this thesis was to estimate surface runoff in irrigated rice systems in eastern Uruguay and evaluate its contribution to N and P export under two contrasting crop rotations. In the first stage, a daily water balance model was calibrated and validated over three consecutive growing seasons. Calibration was performed by comparing estimated and measured water depths in the field, and validation included direct runoff measurements obtained from weirs and flow meters. The results showed strong agreement between estimates and measurements, demonstrating the robustness of the model as a tool to quantify surface runoff. Total runoff—generated by irrigation, rainfall, and final drainage—accounted for between 7.5% and 18% of the total water input to the system. In a second stage, N and P losses associated with surface runoff were quantified in two contrasting rotations: continuous rice (Ri-Ri) and rice–pasture (Ri-Pa). Concentrations of total nitrogen (TN) and total phosphorus (TP) were determined during the different phases of the rice cycle (dry-soil and flooded phases) as well as during the winter cover crop phase, integrating these concentrations with runoff volumes estimated using the validated model. Results showed that the highest TN and TP concentrations were recorded during the first week after flooding of the rice crop; however, the absence of runoff events during this period limited effective export. In contrast, the greatest nutrient loads occurred during the winter cover phase, when large runoff volumes generated cumulative losses of 3.75–5.76 kg ha⁻¹ of TN and 1.31–1.95 kg ha⁻¹ of TP, compared with 1.72–2.01 kg ha⁻¹ of TN and 0.89–1.42 kg ha⁻¹ of TP during the rice growing period.