Türkiye geneli olasılık tabanlı kuraklık analizi
Drought analysis based on probabilistic approach in Türkiye
Yazar: ALİ ALTAŞ
Danışman: DOÇ. DR. İSMAİL DABANLI
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Hidrolik ve Su Kaynakları Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2024
Konu: İnşaat Mühendisliği = Civil Engineering

Özet:
İklim değişikliğinin Türkiye üzerinde belirgin etkilerinin gözlemlendiği doğal afetlerin en önemlileri arasında taşkın, sel, orman yangınları gibi afetlerin yanında kuraklık da bulunmaktadır.  Kuraklık diğer afetlerden farklı olarak sinsi bir afet olarak tanımlanır. Genelde ne zaman başlayacağı ve biteceği belli değildir. Genel olarak kuraklık meteorolojik kuraklık süreci öncelikle başlar, daha sonra toprak nemindeki azalmaya bağlı tarımsal kuraklık, ardından da yer altı ve yer üstü sulardaki azalmalara göre hidrolojik kuraklık başlar.
Kuraklık analizleri indeksler yardımıyla yapılmaktadır. Veri ihtiyacı ve uygulama sadeliği bakımından bu çalışmada SPI indeksi kullanılmıştır. SPI serileri Meteorolojik kuraklıkla ilişki kurmak için 3 aylık periyotlarda, Tarımsal kuraklıkla ilişki kurmak için 6 aylık periyotlarda ve son olarak Hidrolojik kuraklıkla ilişki kurmak için 12 aylık periyotlarda seçilmiştir. 
Türkiye geneli 224 yağış gözlem istasyonunun verileri 1960-2020 zaman aralığında 10'ar yıllık periyotlara ayrılarak incelenmiştir.  224 SPI serisinin Türkiye haritası üzerinde aynı anda gösterilmesinin imkânsız oluşu, Olasılık Tabanlı Alan Dağılımı (OTAD) yönteminin geliştirilmesine öncülük etmiştir. Bu yöntemde her bir SPI kuraklık sınıfı ve Normal ve Islak dönem toplam indeks değerleri, görülme sayılarına göre olasılık değerleriyle ifade edilmiştir. Hesaplanan olasılık değerleri Türkiye haritası üzerinde her 10 yıllık periyotta 4 farklı kuraklık sınıfını kapsayacak şekilde toplamda 4x6=24 adet harita üzerinde gösterilmiştir. Olasılık değerlerinin haritalandırılmasında Kriging yöntemi kullanılmıştır. Analiz sonuçlarına göre kuraklık serileri 1960-1990 ve 1990-2020 olmak üzere genel olarak iki ayrı eğilim ortaya çıkmıştır.  Ayrıca toplam yağışın yüksek olduğu ve yüksek değişkenlik gösterdiği yerlerde olasılık hesapları da değişimin az olduğu istasyonlara nazaran daha fazla olduğu ortaya çıkmıştır.

Abstract:
Drought is among the most important natural events in which significant effects of climate change are observed on Türkiye. Drought is defined as an insidious disaster, unlike other disasters. Generally, it is not certain when it will start and end. It is even noticed after the drought begins. In general, the meteorological drought process begins first, then agricultural drought starts due to the deficiency in soil moisture, and then hydrological drought begins due to the decrease in underground and surface water resources.
In drought assessment, analysis is generally reached with the help of indices. There are several indices in the literature that based on different parameters. The SPI (Standardized Precipitation Index) index was chosen in this study due to data need and simplicity of implementation.  SPI series were created in 3-month scales to describe meteorological drought, in 6-month scales to describe agricultural drought, and finally in 12-month scales to set relationship between precipitation and hydrological drought periods.
The data of 224 precipitation observation stations throughout Türkiye were examined between 1960-2020. To evaluate drought propagation analysis period were divided in to 6 sub-period by 10-year time length.  The hardness of visualizing 224 SPI series simultaneously on the map of Türkiye led to the development of the Probability Based Area Distribution (PBAD) (Olasılık Tabanlı Alan Dağılımı-OTAD) method. In this method, index values of each SPI series are converted to probability scores, taking into account the number of occurrences. The calculated probability scores are shown on the map of Türkiye in a spatial distribution. Totally 24 maps (4x6=24), covering 4 different drought classes in each 10-year period were generated separately by using Kriging methodology.
The variability of drought over time without was examined through its probabilities over 10-year periods. According to the results obtained, it has been observed that the 60-year time series generally exhibits two different behaviors. While the first phase (First 30 years) covers the period 1960-1990, the second phase (Last 30 years) covers the period 1990-2020. These two half periods have a clear increasing trend in drought classes. It has been observed that there has been a serious increase in drought in the last 10 years (2010-2020), especially in the possibility of Extremely Severe Dry Periods. During severe drought periods, a more significant increase is observed in both phases compared to the Moderate Dry period. Although drought is divided into sub-phases over time, it can be said that it generally continues with a stronger upward trend. Considering all drought classes, one of the important results was that 20% of the time was in drought and 80% of the country did not experience any drought conditions during the 60-year period.
The spatial distribution of drought shows a very complex behavior. It can be said that areas of different sizes are similar to each other in the 10-year periods determined in all drought classes, including normal and wet periods. In this case, it can be deduced that drought boundaries cannot be determined clearly, however regionally the probability of drought is higher towards the Southeast, the Mediterranean, Central Anatolia and the Marmara regions. In addition, it can be said that the spatial distribution remains smaller for Extremely Severe Drought conditions or Agricultural Drought evaluations.
Implementing PBAD methodology by using other drought indices found in the literature, may lead to development of the methodology in the future by comparing the results. In the PBAD method, areal distributions are created by taking into account the frequency of drought occurrences. Naturally, stations have very different precipitation regimes can fall between areas with similar characteristics according to the frequency of drought in the PBAD method. In some series, the areas in the Central Anatolia Region, which receives little rainfall, have similar characteristics to the areas in the Eastern Black Sea region, based solely on probability similarity. It would not be correct to define similar climate and rainfall characteristics by looking and comparing at these areas. Inferring that drought likely occurred at the same time in these regions within a 60-year period will lead to the correct analysis outputs.
The maps created regarding the spatial distribution of drought obtained in this study should be evaluated as areas experiencing similar dry or wet processes. Stations with high correlation with similar stations in a row can be considered similar in terms of drought conditions. In this case, the probability of a drought occurring in the region represented by one observation station will also be high in the areas represented by the other observation station with a high correlation. In addition to spatial distribution, drought propagation over time can be evaluated together to understand more clearly which type of drought a region is more exposed to. 
Drought based disaster damage is not only caused by drought hazard. Exposure and vulnerability directly affect the occurrence of drought damages. Drought-related risks are managed by drought risk management. It can only be controlled by reducing exposure and vulnerability. Preventing drought naturally seems impossible, considering climate change and natural circulations. The formation of drought is a natural. However, the damages can be controlled. Damages during droughts can be mitigated with agricultural and hydrological drought storage systems. If exposure and vulnerability analyze are made separately according to drought types and some preventive measures are started to be implemented before the drought, and previously prepared disaster management plans are put into account during the dry period exist. The damages caused by drought can be mitigated from a socio-economic perspective and sustainable disaster risk management approach.