Assessment of pan coefficient models for the estimation of the reference evapotranspiration in a Mediterranean environment in Turkey

Abstract

Reference evapotranspiration (ETo) is essential for irrigation practices and the management of water resources and plays a vital role in agricultural and hydro-meteorological studies. The FAO-56 Penman-Monteith (PM) equation, recommended as the sole standard method of calculating ETo by the Food and Agriculture Organization of the United Nations (FAO), is the most commonly used and accurate model to determine the ETo and evaluate ETo equations. However, it requires many meteorological variables, often restricting its applicability in regions with poor or missing meteorological observations. Many empirical and semi-empirical equations have been developed to predict the ET0 from numerous meteorological data. The FAO-24 Pan method is commonly used worldwide to estimate ETo because it is simple and requires only pan coefficients. However, pan coefficients (Kpan) should be determined accurately to estimate ET0 using the FAO-24 Pan method. As the accuracy and reliability of the Kpan models can be different from one location to another, they should be tested or calibrated for different climates and surrounding conditions. In this study, the performance of the eight Kpan models was evaluated using 22-year daily climate data for the summer growing season in Adana, which has a Mediterranean climate in Turkey. The results showed that the mean seasonal pan coefficients estimated by all Kpan models differed significantly at a 1% significance level from those observed by FAO-56 PM according to the two-tail z test. In the study, ETo values estimated by Kpan models were compared against those obtained by the FAO-56 PM equation. The seasonal and monthly performance of Kpan models was varied, and the Wahed & Snyder model presented the best performance for ETo estimates at the seasonal scale. (RMSE = 0.550 mm d−1; MAE = 0.425 mm d−1; MBE = −0.378 mm d−1; RE = 0.134). In addition, it showed a good performance in estimating ETo on a monthly scale. The Orang model showed the lowest performance in estimating ETo among all models, with a very high relative error on the seasonal scale. (RMSE = 1.867 mm d−1; MAE = 1.806 mm d−1; MBE = −1.806 mm d−1; RE = 0.455). In addition, it showed the poorest performance on a monthly scale. Hence, the Wahed & Snyder model can be considered to estimate ETo under Adana region conditions after doing the necessary calibration.

Estimation of reference evapotranspiration using some class-A pan evaporimeter pan coefficient estimation models in Mediterranean–Southeastern Anatolian transitional zone conditions of Turkey

“…Cuenca >Raghuwanshi & Wallender>Allen & Pruitt>Pereira>Orang >Snyder. In another study conducted in Mediterranean climate conditions, Koç (2022) reported that Wahed & Snyder was the best performing model (MAE = 0.43 mm day −1 , RMSE = 0.55 mm day −1 ) and Orang was the worst performing model (MAE = 1.81 mm day −1 , RMSE = 1.87 mm day −1 ) in Adana, 195 km from Kahramanmaraş. The accuracy ranking of the eight models discussed in this study, was as follows.…”

“… Aschonitis, Antonopoulos & Papamichail (2012) concluded that the models with the best and worst estimating performances were Cuenca and Snyder, respectively, in their study realised in the Thessaloniki Plain of Greece, which has a semi-arid Mediterranean climate. Koç (2022) stated that in Adana, located in southern Turkey with a hot-summer Mediterranean climate, the models with the best and worst estimating performances were Wahed & Snyder and Snyder, respectively. It has been observed that the reliability and usability levels of the K p coefficient estimation models evaluated within the scope of these studies vary depending on climatic and environmental conditions.…”

Comparison of the Performances of Pan-Based Equations to Estimate Reference Evapotranspiration (ETo) in Mediterranean Climate Conditions

“…Tüketimini (ETo) Tahmin Etme Performanslarının Karşılaştırılması Giriş Kıyas bitki su tüketiminin (ETo) doğru olarak tahmin edilmesinin, su kaynaklarının yönetimi, sulama uygulamaları, tarımsal ve hidrometeorolojik çalışmalar ile farklı hidrolojik süreçlerin modellenmesi çalışmalarında çok önemli bir yeri vardır (Koç, 2022) Bu yaklaşımın en önemli avantajı ise düşük maliyetli olması ve sulama programlamasında kullanımının kolay olmasıdır (Stanhill, 2002). Kap buharlaşmasının (Epan) ETo'ya dönüştürülmesinde en çok kullanılan eşitlik, FAO24 Pan ETo eşitliğidir (Doorenbos ve Pruitt, 1977).…”

“…Kap buharlaşmasının (Epan) ETo'ya dönüştürülmesi amacıyla, Kpan eşitliklerinin değerlendirildiği birçok çalışma yapılmıştır. (Grismer ve ark., 2002;Irmak ve ark., 2002;Sentelhas ve Folegatti, 2003;Gundekar ve ark., 2008;Sabziparvar ve ark., 2010;Aschonitis ve ark., 2012;Tabari, ve ark., 2013;Koç, 2022). Ancak, literatürde Akdeniz iklimi koşullarında yapılan sadece birkaç çalışma bulunmaktadır (Grismer ve ark., 2002;Aschonitis ve ark., 2012;Koç, 2022).…”

“…(Grismer ve ark., 2002;Irmak ve ark., 2002;Sentelhas ve Folegatti, 2003;Gundekar ve ark., 2008;Sabziparvar ve ark., 2010;Aschonitis ve ark., 2012;Tabari, ve ark., 2013;Koç, 2022). Ancak, literatürde Akdeniz iklimi koşullarında yapılan sadece birkaç çalışma bulunmaktadır (Grismer ve ark., 2002;Aschonitis ve ark., 2012;Koç, 2022). Grismer ve ark (2002) Not: Tort = aylık ortalama sıcaklık, Tmax = aylık ortalama en yüksek sıcaklık, Tmin = aylık ortalama en düşük sıcaklık, n = aylık ortalama güneşlenme süresi, Pn = aylık ortalama yağışlı gün sayısı, P = aylık ortalama toplam yağış miktarı, N = Nisan, M = Mayıs, H = Haziran, T = Temmuz, A = Ağustos, Ey = Eylül, Ek = Ekim.…”