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Performance of DRAINWAT Model in Assessing the Drainage Discharge from a Small Watershed in the Po Valley (Northern Italy)

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  9th International Drainage Symposium held jointly with CIGR and CSBE/SCGAB Proceedings, 13-16 June 2010  IDS-CSBE-100086.(doi:10.13031/2013.32116)
Authors:   Maurizio Borin, Tomaso Bisol, Devendra M Amatya
Keywords:   Agricultural N losses, Watershed, Drainwat, Italy

Evaluation of potential nitrogen (N) losses from individual fields is not sufficient to provide an estimate of the actual nitrogen loads reaching the main watercourses and, therefore, it is becoming a relevant source of pollution. Along the travel path from a field to the outlet of a watershed several biogeochemical processes may occur, leading to significant changes in the N amount actually leaving the watershed. These processes can be described using models of various complexities including lumped exponential decay model, which uses travel (residence) time from predicted velocities in the ditch-canal network. However, hydrology is a driving variable for accurate predictions of nutrient loadings. A case study was conducted on Longhrola watershed, located in the Mantova province, Po valley, Northern Italy, where hydrologic measurements and N input at field level, N harvested with crops and N lost at the outlet of the basin were recorded during 2002-2005. The 65 hectares watershed has its land use as crops for livestock. Liquid manure and farm-yard manure are available in abundance and are the main source of nitrogen for crop fertilisation. At basin scale, in the three years of monitoring, the average yearly N input was 130 kg ha-1, the losses at the outlet of the basin ranged 8 to 24 kg ha-1 year-1 depending upon the rainfall amount and distribution. A watershed-scale hydrology model, DRAINWAT, applied to predict the stream outflows for this watershed showed results (522 mm) consistent with the measured data (549 mm), which was only a 4% underprediction for calibration (2002-04) period. However, there was an underestimation (236 mm) of 11% compared to the measured (265 mm) data in 2004-05 validation period. This was considered acceptable, especially considering the minimal field measurements, for its future application in estimating cumulative load using a simple nutrient model with travel time.

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