Evaluating The Relationship Between Cumulative Rainfall and Yield of Wheata and Barley Using a Evenness Index in The Semi-Arid Region of Mashhad

Rainfall pattern in any location has its own distribution, while visual analysis of that can lead to subjective and vague statements. So, finding usefull indices which can explain rainfall distribution is an urgent to better interpret rainfall pattern. Some indices are calculated based on the distance between cumulative rainfall curves and the respective theoretical evenness rainfall lines, which join the lowest and highest cumulative rainfall values within the considered period. In present study the relationship between the indices and yield of two major crops (wheat and barley) are evaluated over a 30-year period (1984-2013) in Mashhad. The indices were applied for two periods (the planting to the harvest of crop and critical stage). Critical stage was limited to 30 days from May-Jun. Results showed that both crops were significantly related to ER (the slope of the evenness line) (up to 0.5), while they were not related to UR (uneven distribution index rainfall). When the evenness index was weighted on the mean daily rainfall, the slope of the evenness line declined (up to 0.45). Significant correlations were also found by calculating the indices over strategic short-term periods for crops (up to 0.48), which indicates that how the effectiveness of these indices may be improved by choosing appropriate periods for different crops.