Türkiye’deki 2000-2015 yılları arasındaki buharlaşma ve terlemenin NOAH hidrolojik modeli ile incelenmesi

Abstract

Kuraklığın incelenmesi, etkilediği alanlarda büyük zararlara sebebiyet veren bu doğal afetin niteliklerinin (şiddet, süre, etki alanı) anlaşılması ve mümkün olan tedbirlerin araştırılması açısından çok kritiktir. Kuraklığın gözlemlenmesinde kullanılan en önemli parametrelerden birisi olan buharlaşma ve terleme (ET) verisidir. ET tanımı, yaş bitki ve toprakta meydana gelen buharlaşma ile kuru bitkilerde meydana gelen terleme sonucu kaybedilen suyun toplamı olarak yapılabilir. Su ve enerji döngüsünde önemli bir yere sahip olan ET verisi farklı metotlar kullanılarak elde edilebilmektedir. Arazide yapılan ölçümler, hidrolojik modeller ve uzaktan algılama ile yapılan gözlemler ET verisini elde etmek için yaygın olarak kullanılan metotlardır. Hidrolojik modeller zamansal ve mekânsal olarak sürekli ve tutarlı veri sağladıklarından bu modellerden elde edilen ET verileri kuraklık incelemelerinde yaygın olarak kullanılmaktadır. Bu çalışmada, NOAH hidrolojik yeryüzü modelinden elde edilen ET değerlerinin 2000 – 2014 yılları arasında ülkemizdeki değişimi ve ülkemizi etkisi altına alan kurak koşulların etkileri ayrıntılı bir şekilde incelenmiştir. Yıllık olarak yapılan inceleme ile çalışma için seçilen yıllar arasında ülkemizdeki kurak ve ıslak geçen yılların ET değerleri kullanarak belirlenmesi amaçlanmıştır. Ayrıca aylık olarak yapılan inceleme ile 2014 yılında ülkemizde yaşanmış olan kuraklığın diğer yıllardaki ayların da kullanımı ile belirgin bir şekilde gözlemlenmesi sağlanmıştır.

Keywords

• Buharlaşma ve terleme, Hidrolojik Modeller, Kuraklık

Analysis of evapotranspiration over Turkey in years between 2000 and 2015 using NOAH hydrological model

Abstract

Analysis of drought, which causes substantial damage to affected areas, is critical in terms of understanding this natural disaster’s characteristics (intensity, duration, influence area) and taking necessary precautions. Evapotranspiration (ET), one of the most critical agricultural drought indicators, can be retrieved using different methods. Among them hydrological models offer a great advantage via its potential of providing continuous and high spatial and temporal resolution ET data. NOAH land surface model is one of the most widely used hydrological models to retrieve land surface parameters and variables like soil moisture and temperature for different layers of soil, ET, runoff, and sensible heat. Model simulations require availability of various atmospheric forcing data and land surface parameters that can be obtained from different sources at various resolutions (Table 1). In many cases spatial resolution of the input forcing data largely determine the spatial resolution of the output variable. For this study, NOAH ET simulations are obtained at 0.25 spatial and 3 hour temporal resolution between 2000 and 2014. Retrieved 3-hourly ET products are later averaged into daily estimates which are used in this study. Overall, lower ET values over extended durations are often associated with an ongoing agricultural drought conditions while higher values signal wet periods. Spatial distribution of the daily ET estimates over entire Turkey is consistent with the expectations that mean ET values of the Central Anatolian Region are lower than rest of the Turkey and the coast line of the Eastern Black Sea Region has higher mean ET estimates and most part of the Aegean Region has ET values with similar magnitude to the mean values over the country. Land surface processes over Turkey show strong seasonal cycle: winters are wet and cold while summers are dry and hot. As a result, day-to-day variations of daily ET values are much higher than inter-annual variations. This implies the retrieved ET variability is dominated by the seasonality of this variable. Hence, it is necessary to remove this strong seasonality signal of land surface processes to analyze the surface conditions. For this reason, 29-days moving window averages of ET over each pixel are calculated separately as the seasonality of this particular pixel (Eq. 1). Removal of these seasonality information give the ET anomaly information which is the primary associated with drought. ET anomaly time series obtained over a pixel in Şanlıurfa – Ceylanpınar (in South Eastern region of Turkey) is presented to give an example about the capability of NOAH ET simulations to reflect the local conditions. These time series show ET anomalies in 2013 are the highest while the summer of 2008 shows an extended duration of below normal conditions. Drought investigations often involve normalized values to remove the impact of the local climate. Accordingly, 15 year monthly averages and standard deviations are used to normalize monthly ET data. Such normalization helps intercomparison of the impact of the drought over different locations directly. Results show 2000 can be defined as most wet year nearly all over the country. On the other hand, 2007 and 2014 years also passed under the influence of a dry period compared with the other years. The change in the severity and the areal extent of 2014 drought is monitored by analyzing the monthly normalized ET maps between July 2013 and September 2014 in detail over the study region. April of 2014 is the driest April in 15 years where drought impacted severely over large regions in May and June 2016. The drought seemed to lose its severity in August over most parts of the study region.

Keywords

• Evapotranspiratıon, Hydrological Models, Drought