A study of the relationship between drought and solar irradiation variables in Yazd station

Drought prediction, in many regions of the world, is critically important for strategic planning of agriculture and water management. The occurrence of droughts cannot be predicted with certainty and, thus, must be treated as a random variable. It is the main purpose of this paper to develop a new concept of drought and assessment of its features based on triple drought-related hydro-meteorological and meteorological variables, namely rainfall, solar irradiation and sunshine duration. At first, the scatter diagrams of rainfall, solar irradiation and sunshine duration are drawn as prescribed by the Dual Graphic Method (DGM). Then, the monthly drought conditions that are observed are compared together on the basis of the SMP values table. Also, equal standard rainfall lines are drawn as a map with two reference variables including solar irradiation and sunshine duration. These are referred to as the triple graphical method (TGM). It furnishes rich features of drought behavior variation based on the rainfall, solar irradiation and sunshine duration. This analysis and the solar irradiation estimation method are applied to Kermanshah stations in the Republic of Iran based on monthly data obtained within the period of 1970–2005. The linear relationship between drought and the variables of the Angstrom equation is found to be 0.80. Also, in severe drought conditions, the irradiation variables of H/H_o and S/S_o are found to be varied in the range of 0.66-0.79 and 0.71- 0.86 respectively