Hydrologic Variables 12221 Streamflow

Streamflow represents the integrated response of a watershed to precipitation and evapotranspiration (partly driven by temperature) processes after being filtered by watershed physical characteristics and, in some cases, human-induced factors. Due to this close link between global warming and streamflow response of a watershed, several studies focused on possible trends in Turkish streamflow records.

The regional patterns emerging from these studies are in general parallel to the trends in atmospheric variables. Most consistent pattern is the decreasing behavior in streamflow in western parts (Marmara and Aegean) [5, 11, 20, 22, 24-28]. Similar behavior has also been reported for Central Anatolia and southern parts [24, 27, 28]. In other regions of Turkey no consistent trend has been reported, except a few stations with increasing streamflow trend in the north [11, 28].

The trans-boundary Tigris-Euphrates (TE) River system located in eastern Turkey is fed by seasonal snowmelt and is critical for a series of socio-economic development projects in irrigation and energy production sectors. Few studies [5, 33] have reported significant decrease in low flow conditions for many stations in eastern Turkey. In the Upper Euphrates basin, insignificant decreasing trend in annual average runoff and slight earlier melting of snow for the period 1994-2004 was detected [10]. Snow data could provide further insight into the streamflow potential of the TE system. For example, Surer et al. [30] investigated the snow cover variation in Karasu basin during 2000-2009 using remote sensing (MODIS). Although no apparent change in snow cover was detected, the runoff significantly decreased during 2008-2009. One possible explanation is the reduction in snow water equivalent due to the decrease in the amount of precipitation over the years. Groundwater

Relative to surface water resources, the potential consequences of climate change on groundwater have not received as much attention. Groundwater reacts to climate change due mainly to changes in recharge characteristics. Considering that the precipitation and runoff exhibit decreasing trends over many regions in Turkey, it is likely that groundwater levels in these regions will reveal decreasing behavior, perhaps generally with a time-lag. This situation will be further intensified due to increasing water demand. Negative consequences include: decrease in recharge and hence in groundwater levels, deterioration in water quality due to increased demand, sea water intrusion along coastal aquifers and salinization due to increased evapotranspiration.

The trend studies on precipitation, temperature and streamflow time-series mostly agree that basins located in western Turkey are highly vulnerable to impacts of climate change. The groundwater vulnerabilities in this region could be revealed by an investigation of the atmospheric, hydrologic and water use data for the Kucuk Menderes River basin [31]. Groundwater is the dominant source of agriculture in the basin. Temporal variations in precipitation (Odemis station), streamflow (Selcuk station) and baseflow in the basin are shown in Fig. 12.1. The figure shows that streamflow (and baseflow) fluctuates in response to general trends in precipitation. Now focusing on the groundwater system, Fig. 12.2 shows the temporal variations in water-table depth in an observation well near Torbali and average yields and depths of the registered private wells in the basin. Comparison of Figs. 12.1 and 12.2 reveals a close link between time variation in streamflow and water table depths due to changes in recharge conditions. Another factor responsible for declining water table is the significant increase in groundwater pumping. It is estimated that over-utilization of groundwater resources initiated in the early 1980s [31].

Fig. 12.1 Temporal variations in precipitation, streamflow and baseflow in the Kucuk Menderes River basin, Western Turkey. Precipitation data is from Odemis meteorological station and stream-flow data is from K. Menderes River Selcuk station (Modified from Yilmaz [31])

1968 1985 1998

Fig. 12.2 Temporal variations in water table depth, average well yields and depths of private wells. Water table data is from an observation well near Torbali (Modified from Yilmaz [31])

1968 1985 1998

Fig. 12.2 Temporal variations in water table depth, average well yields and depths of private wells. Water table data is from an observation well near Torbali (Modified from Yilmaz [31])

It can be seen from Fig. 12.2 that, drastic increase in water table-depth initiated in the early 1980s making deeper wells necessary. More importantly, the average well-yields declined in response to increase in well-depths. It is expected that, with global warming, climate will become drier; the water tables will continue to drop in response to both decrease in recharge (reduced surface water) and increase in dependence on groundwater. Together with the predicted rise in sea levels, already existing sea water intrusion problem [32] will be exacerbated, further deteriorating water quality along the coastal aquifers.

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