Spatio-Temporal Variability of Evapotranspiration over the Kingdom of Saudi Arabia

Climate change and variability is receiving much attention recently. Changes in the world temperature and other natural phenomenon such as rainfall variability are being considered as indicators to climate change. These changes can have significant effect on water resources and therefore on the livelihood of society especially in water scarce countries such Saudi Arabia. The Kingdom of Saudi Arabia suffers a chronic water scarcity, despite the fact that the agricultural sector consumes around 90% of the water budget in the kingdom. The objective of this article is to study the variability in reference evapotranspiration (ETo) demand over the kingdom during the period from 1980 to 2008 using Penman-Monteith formula and to identify possible changes in the ETo as an indicator to climate change in the region. Studying ETo can be an incomparable indicator for climate change because it involves various parameters including temperature, humidity, and wind speed. Identifying changes in ETo can also help in future planning of agriculture-water projects and identify lower and higher ETo zones for proper planning and management of agricultural projects in arid region. Results indicated that the average ETo varied from 5 mm/d in January to more than 15 mm/d in July with extreme value ranged from as low as 1.6 to 30 mm/d. The lowest ETo values were found in the southern region of the Kingdom during the month of November to February and starts to shift to the western region from the month of March to October. The results also indicated that the annual maximum and minimum daily averages of ETo steadily increased over the study period. The ETo annual daily average had increased from about 9.6 mm/d in 1980 to as high as 10.5 mm/d in 2008. Data analysis also revealed that the percentage areas of the Kingdom with high values of ETo depth, greater than 4000 mm/year, have also steadily increased in the study period from 25% to about 40%. On the other hand, areas having lower ETo values, less than 3600 mm/year have decreased over the studied area from about 30% in 1980 to as low as 12% of the kingdom total area. These results indicated clearly an increasing trend in the ETo over Saudi Arabia.