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Title of Journal: Environ. Process.

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Effect of Land-use Change Scenarios on Nutrients and TSS Loads

Authors: Yael Gilboa, Gideon Gal, Doron Markel, Alon Rimmer, Barry M. Evans, Eran Friedler,

Publish Date: 2015/08/28
Volume: 2, Issue:4, Pages: 593-607
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The projected growth in population in Israel by 50 % by 2030 will greatly enhance urban density and motivate increased urbanization of rural regions in the country. The Lake Kinneret watershed is a rural region of which only about 3 % of the total area is used for residence, and currently, it is the least populated region in Israel. A significant land-use change and growth of urbanized regions is therefore expected in the near future, leading to changes in water quality management in the watershed. In this study, we attempted to quantify the effects of these possible changes in land-use on the flow and pollutant loads discharged from the watershed into Lake Kinneret. To that end, we calibrated and verified the AVGWLF (ArcView (GIS) Generalized Watershed Loading Function) model to simulate stream flows, sediment and nutrient loads under the conditions of a Mediterranean climate watershed. In addition to AVGWLF model, we used two external tools, namely: the HYdrological Model for Karst Environment (HYMKE) to predict daily flows of streams which were not simulated by the AVGWLF model, and a Mediterranean Multiplication Factor (MMF) which was used to improve sediment transport and nutrient load simulations. The combined suite of tools successfully simulated the observed data (r2 > 0.70 and Nash-Sutcliffe efficiency >0.69 for flowrate, sediment and nutrient), including extreme values. The successful combination of the models provides watershed and lake managers the ability to examine the potential land-use changes and their impact on the watershed and the lake downstream.The Lake Kinneret watershed is a rural region, and as part of the aforementioned solution in the future it is plausible to assume that parts of the pastures, croplands, and animal farming and grazing area, which are currently the major land-uses in the watershed, will be converted to high-density development urban areas. As these farming practices can have significant impacts on the water quality leaving the watershed (Berman 1998; Wang 2001), it is important to understand the likely impact of the anticipated changes in land-use. These changes are likely to affect the quality and quantity of the water flowing from the watershed into Lake Kinneret.Lake Kinneret is the only large natural freshwater lake in Israel. Being such, it has a unique ecological value; it is an important focal point for water related tourism and a significant source of potable water. Thus, preserving the lake’s ecology is crucial. To achieve this, all activities in the watershed, existing and possible future ones, must be seriously and carefully examined in relation to their potential effects on the lake. This work addresses this issue by assessing the possible effects of four future scenarios on the nutrients and sediment loads discharged from the watershed into the lake.Many distributed models have been used to simulate runoff generation and pollutant loads in Mediterranean watersheds, which is characterized, in general, by two distinct seasons: a dry, long, hot summer and a short rainy winter with a small number of flood events. Flood events are mainly associated with intensive, short-term rainfalls; high runoff rates occur very quickly and then just as quickly return to low base flow values (Bisantino et al. 2013). As typical for Mediterranean climate watersheds, the pollutant loads are predominantly contributed by a limited number of intense flood events, whereas the contribution of all of the other events is negligible. While somewhat limited, a number of studies have applied catchment level models to Mediterranean watersheds (Gikas et al. 2006; Pisinaras et al. 2010; Licciardello et al. 2007; Bisantino et al. 2013; Chamoglou et al. 2014; Oroud 2015). These studies indicated that the model predictions were suitable for runoff and satisfactory for the sediment and nutrient loads.Hydrological modeling of rainfall – stream flow relations in the Lake Kinneret watershed have also been examined in several studies (see review in Rimmer and Givati 2013). The most recent and updated model (Rimmer and Salingar 2006) simulated daily flows, originating from the karst region of the Hermon Mountain (north of Lake Kinneret watershed) as a function of daily precipitation and potential evaporation. However, only a limited number of models have been used to simulate pollutant loads in parts of Lake Kinneret watershed. In the most recent, Preis and Ostfeld (2008) presented a data driven modeling approach for flow and selected pollutant load predictions in a small sub-catchment within the Lake Kinneret watershed (Meshushim). The methodology comprised a coupled model-tree – genetic algorithm scheme. It produced a good fit in most cases, but had limited success in estimating peak flows and water quality loads.These examples emphasize the need for a watershed model to efficiently predict both water quantity and essential water quality parameters such as sediments and nutrient loads for the Kinneret watershed, as part of an effort to assess the effects of possible future scenarios on water quality flowing downstream from the watershed to the lake. The watershed model we used for this study was the “ArcView Generalized Watershed Loading Function” (AVGWLF) model (Evans et al. 2002). The model provides an interface between ArcView geographic information system (GIS) software and the Generalized Watershed Loading Function (GWLF) model (Haith and Shoemaker 1987). GIS technology provides the means for compiling, organizing, manipulating, analyzing, and presenting spatially-referenced model input and output data. The model provides the ability to simulate runoff, sediment and nutrient (N and P) loadings from a watershed given variable-size source areas and land-uses (e.g., agricultural, forested, and developed; Evans et al. 2008). It is a continuous simulation model which uses daily time steps for meteorological input and water balance calculations. Sediment transport is calculated on a daily basis, while monthly stream bank erosion and nutrient loads are calculated based on daily water balances accumulated to monthly values. The AVGWLF model includes special algorithms for calculating the contribution of contaminating point sources: septic systems; nutrient loads associated with farm animals (Evans et al. 2008). The model has been tested extensively and successfully in the U.S. (Evans et al. 2008; Tu 2009; Georgas et al. 2009), Mexico (Carro et al. 2008), and Canada (Georgas et al. 2009).In this study, we calibrated and verified AVGWLF to the Lake Kinneret watershed. We integrated AVGWLF model with two tools (HYMKE and MMF) to better represent unique hydrological characteristics of the specific region. The first tool was the HYdrological Model for Karst Environments (HYMKE; Rimmer and Salingar 2006), which was used for simulating daily spring flow in the Kinneret watershed as function of daily precipitation. The second tool is a Mediterranean multiplication factor (MMF) which was used to improve the prediction of sediment and nutrient load transport. The justification for using HYMKE and MMF is described below. Finally, we used the calibrated and verified combined AVGWLF-HYMKE-MMF tool to analyze the effects of four possible extreme watershed management scenarios on the quantity and quality of water leaving the watershed and entering Lake Kinneret.



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