Nutrient Transport Modelling in the Daugava River Basin - DiVA Portal

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simulation system CatchmentSim. The purpose of the study is also to evaluate themodel and the simulation system and point out strengths and weaknesses that should beconsidered when applying the model to the rest of the Baltic basin.2. BACKGROUND2.1 EUTROPHICATION OF THE BALTIC SEANutrients such as nitrogen and phosphorus are vital to life on land and in water.Moderate input of nutrients to water systems may be beneficial for fish catches since itwill increase the growth of macrophytes. However, an overfertilization will lead to anexcessive growth and the bacterial decay of organic matter may lead to oxygendepletion, especially at the deep bottoms of the sea. Due to this there will be analteration of the ecological community structure. Some species will be favoured whileothers will be disadvantaged in the altered environment. This is called eutrophication(Clark, 2001). Phosphorus is usually the limiting nutrient in fresh water but also in someparts of the Baltic Sea, for instance the Bothnian Bay. In saline water, nitrogen isgenerally the limiting nutrient, which is the case in most parts of the Baltic Sea(BOING, 2002). The anthropogenic sources of nitrogen and phosphorus are leakagefrom agricultural land and forested land, point source discharges from waste watertreatment plants and industries, leakage from septic systems, deposition from theatmosphere and release of phosphorus from the sediments during anoxic conditions(SMHI, 2005).Eutrophication has long been considered to be one of the most serious threats to theBaltic Sea environment. The nitrate concentration in the Baltic Sea south of the ÅlandSea increased almost threefold from the 1960’s to the 1980’s. A pronounced increase ofplankton production has been noticed in this part of the Baltic Sea and the area ofanoxic bottoms has likewise increased. Nowadays, about one third of the bottom area ofthe Baltic Sea is anoxic (SWEPA, 2003) and anoxic bottoms are found in more shallowareas than earlier. Since the 1970’s, when the problem of eutrophication of the BalticSea became apparent, a lot of efforts have been made by the countries surrounding thesea in order to master the problem and reduce the nutrient loads (HELCOM, 2004). Forexample, in Sweden the regulations for storage and spreading of manure and for theproportion of cultivated land that has to be vegetationcovered during fall and springhave been sharpened. Artificial wetlands have also been constructed in agriculturalareas and waste water treatment plants in southern Sweden have introduced nitrogenpurification (SWEPA, 2003). However, in spite of all the efforts the nutrient loads fromland have not decreased significantly. One reason for this is probably the long retentiontime of nutrients in agricultural soils. It is evident that it will take a long time until wewill see an improvement of the situation in the Baltic Sea, but by active and coordinatedmeasures taken by the countries in the region this time can be shortened (HELCOM,2004a).2
2.2 FEATURES OF THE NITROGEN AND PHOSPHORUSCYCLES2.2.1 Nitrogen retention and leakageNitrogen interacts with the atmosphere through nitrogen fixation and denitrification.Nitrogen fixation is the process where certain microorganisms convert nitrogen gas ofthe atmosphere to nitrogencontaining organic compounds. Denitrification, on the otherhand, is the process where nitrate is converted to gaseous forms of nitrogen by a seriesof biochemical reactions. Nitrogen in the watershed may thus leave the system throughdenitrification before it reaches the mouth of the river. It is difficult to predict themagnitude of denitrification, but 5 to 20 % of the nitrogen in some streams may be lostby nitrification. Due to this nitrogen retention the net amount of nitrogen that reachesthe sea is smaller than the gross amount that has been added in the watershed (Bradyand Weil, 2002).Nitrate is generally the predominant mineral form of nitrogen in most soils. Since thenitrate ion is negatively charged it is not adsorbed to the negatively charged colloidsthat dominate most soils. Therefore, nitrate ions easily move downward with drainagewater and are readily leached from the soil into the groundwater. The leaching ratedepends on the rate of percolation through the soil, the concentration of nitrogen in thedrainage water and the texture and structure of the soil. Great nitrogen losses may comefrom agricultural land if the input of nitrogen exceeds the amount removed by harvest,but with proper management the losses can be reduced (Ibid).2.2.2 Phosphorus leakage and erosionUnlike nitrogen, phosphorus is not lost from the soil in gaseous form. Leaching lossesare also generally very low since soluble, inorganic forms of phosphorus are stronglyadsorbed by mineral surfaces. However, leaching losses may still be large enough tocontribute to eutrophication in downstream waters. The more common pathway ofphosphorus leaving the soils is through erosion of phosphoruscarrying soil particles(Ibid).2.3 GENERAL MODEL DESCRIPTIONThe Generalized Watershed Loading Functions (GWLF) is a mathematical model thatestimates monthly loads of nitrogen and phosphorus from watersheds. Themathematical structure of the model is simple. Surface runoff, groundwater flow andpoint sources of nutrients are included in the model. Examples of required input to themodel are climate data and information on land use and soil types in the basin. Theoriginal GWLF model was described by Haith and Shoemaker in 1987 (Dai et al.,2000). An overview of the hydrological part of the GWLF model is shown in figure 1.In this study the computer program CatchmentSim has been used. CatchmentSim is aWindows based simulation system for simulating nutrient yields from watersheds. Thesystem is based on a modified version of the GWLF model and it has been developed atStockholm University (Mörth, 2004). From now on in this report GWLF will refer tothe modified version of the GWLF that has been used in the study if nothing else isstated.3
Page 1 and 2: UPTEC W05 009Examensarbete 20 pFebr
Page 3 and 4: Nyckelord: avrinning, näringsämne
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Page 10 and 11: Figure 1. The hydrological part of
Page 12 and 13: TemperaturePrecipitation20.00120deg
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Page 16 and 17: Groundwater discharge to the river
Page 18 and 19: Groundwater nutrient loadsThe groun
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Page 22 and 23: a) Streamflow, calibration periodb)
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Page 26 and 27: the chosen value is a very rough ap
Page 28 and 29: all cultivated land will have winte
Page 30 and 31: HELCOM, 2004a. 30 years of protecti
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Page 34 and 35: Description of parameter/input Name
Page 36 and 37: Description of parameter/input Name
Page 38 and 39: moisture.Figure B1. Selection of cu
Page 40 and 41: wherea t = rainfall erosivity coeff
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