Bendrojo azoto ir bendrojo fosforo apkrovų ir sulaikymo kaita Mūšos upės baseine, apskaičiuota taikatnt FyrisNP 3.1 modelį ; Quantifications of the total nitrogen and total phosphorus loads and retention changes in the Mūša river catchment using FyrisNP 3.1 model

The paper presents the data-oriented FyrisNP 3.1 model based quantification and discussion of total nitrogen (NB) and total phosphorus (PB) transport, retention, loads and model reliability in the catchment of the River Muša and its tributaries during 2000-2010. NB and PB model effectiveness is 0.42 and 0.31, correlation coefficient - 0.65 and 0.58 and relative error - 46.0 and 37.2 %, correspondingly. Model source apportionment calculation assigns 91.6 and 6.1 % of the total NB load to agriculture and point sources correspondingly, while 42.2 % of the PB is assigned to agriculture and 38.9 % to point sources. Calculations show negligible increase of agriculture load and significant decrease of point source load during the investigated period. The lowest specific NB and PB loads are in the sub catchment L vuo upstream Kupiškis – 5.1 and 0.035 kg ha-1 correspondingly and the highest - in the Tatula sub catchment at Trecionys 12.7 and 0.08 kg ha-1. NB and PB retention in streams and lakes is estimated, correspondingly, 22.2 and 11.5 %. Comparison of the result of two periods (1997-2006 and 2000-2010) and different number of sub catchments has determined no change of load form agriculture, and increase of PB load by 4.1 %. Reduction of monitored catchments and transfer of monitored sites upstream increases relative error by 24.8 and 7.0 % for NB and PB correspondingly. To increase model accuracy it is necessary to establish water quality measurement sites in outlets of all larger tributaries and take composite samples 2-4 times per month during peak flow and once a moth in the rest of the year.