**2.3.1 Watershed models**

Watershed models are derived from hydrological models, usually from a formerly known hydrological model so that they contain all the key hydrological processes. They also contain sediment transport and terrestrial biogeochemical cycle related processes.

Since 1960's many hydrological models were developed and some of them evolved to general purposed watershed models. However, just a minor fraction of them were designed to simulate the hydrology and ecology of entire watershed using the coupled modelling approach and even fewer of them were continuously developed and became widely used, freely available open-source modelling tools. SWAT (Arnold et. al., 1999) and HSPF (Brickner et. al, 2001) are good examples for such modelling tools. WASH123D (Yeh et. al, 2005) is more comprehensive than SWAT and HSPF as a hydrological model but has simpler water ecology related facilities. MIKE-SHE is also capable to simulate the watershed hydrology and ecology; however it is neither free nor open source; and needs additional products such as MIKE-11 to be coupled with. Other models such as SWAP (Van Dam, 2000; Van Dam et al., 2008), PIHM, Hydrogeosphere (Therrien et al, 2010) are general hydrological model taking almost all the compartment of the hydrological and can be easily linked to landscape and water ecology models.

Among all the models discussed SWAT and HSPF are the most widely used ones and generally most applicable ones. The applicability of SWAT was reviewed by Gassmann et al

Managing the Effects of the Climate Change on Water Resources and Watershed Ecology 265

ecosystem models represent the individuals by the changes in their numbers; hence, they called as population models. However, AQUATOX simulates the ecosystem by changing the concentrations of all components such as chemicals, sediments, and even organisms including the ones on the higher trophic levels of food web. The model is intended to assess dynamic effects of various stressors such as temperature, toxic chemicals, nutrients, sediment; which

is applied to aquatic environments from experimental tanks to lake systems.

from a couple of weeks up to a couple of years depends on following factors:

• operating schedule (in case of engineered systems)

**3.1 Change of water quantity reaching the water resources** 

• the intensity of the effect • internal structure of the system

season may decrease as well.

droughts.

**3. Effects of climate change on water resources and watershed ecology** 

Climate change may have short and long term effects on watershed ecosystems resources. Short term effects take place because of the extreme events that are related to climate change. Floods are good examples for such extreme events. During a flood shock loading of sediments, organic matter and nutrients can be transported into lentic freshwater ecosystems such as lakes and reservoirs. Aquatic ecosystems respond to such sudden forcing by instantaneous changes in water quality. Recovery of the system that may take

Long term effects on water resources occur due to climatic trends and extended periods of

The relation between the components of the historical water balance and climatic variables may be needed as reference in order to quantify the effect of climate change on the water balance of a watershed. This task is straightforward if historical data on both; the climatic variables and the water balance components exist. If one of them is missing the other one can be reconstructed using simulation techniques. Kavvas et al., (2009) used a regional hydro climatic model (RegHCM-TE) for Tigris-Euphrates watershed located in the Middle-East for reconstructing the historical precipitation data to perform water balance computations for infiltration, soil water storage, actual evapotranspiration and direct runoff.

Climate change may result in average temperature and total precipitation increase. However the temporal and spatial heterogeneity of meteorological parameters may increase as well resulting in prolonged dry season and increased in flood frequencies in wet season. Average temperature in the warm season may increase and average temperature in the cold

Changes in precipitation and temperature do not only change the total amount of runoff to freshwater systems from their catchments but also the temporal distribution of water inputs. Generally, intensification of the global hydrological cycle is expected as a result of temperature increase. However, if the land surface hydrology is dominated by the winter snow accumulation and spring melt, temperature increase is likely to cause a change in the outflow hydrographs of the watersheds where time of peak flow will be shifted towards winter. Detailed information related to this phenomenon is provided by Barnett et al., (2005) in great detail. Forbes et al., (2011) analyzed the water cycle in a small snow dominated

(2007). There is also a literature database on SWAT website, which indicates that SWAT and its variants were applied 816 times in studies published by peer reviewed journal articles reporting hundreds of applications on different watersheds all over the world. HSPF on the other hand is widely used as well. The bibliography provided by developers contains more than 300 entries. The performance of SWAT and HSPF were compared by several authors (Im et al., 2003; Nasr et al., 2004; Saleh and Du, 2004; Sigh, et al., 2004), where both models were applied to the same watersheds. In these studies, both models produced comparable results; however HSPF produced slightly more accurate results in river discharges, whereas SWAT was better in reproducing the nutrient loads.
