A simple method to improve the estimation of the relationship between rainfall and crop yield

The time-course of rainfall is commonly presented as bar, line or scatter plots, which may sometimes be chaotic, while leading to subjective and vague assertions. More objective criteria quantifying rainfall distribution may therefore be helpful, e.g. when different years are examined concurrently. Here, some practical indices are presented based on the distance between cumulative rainfall curves and the respective theoretical evenness rainfall lines, the line joining the lowest and highest cumulative rainfall values within the considered period. After an introduction to the theory and calculation of the indices, the relationships between the indices and two major crops, maize and sugar beet, are evaluated for a period of 33 years. The results show good correlations between the indices and crop yields, up to r = 0.81, especially when the evenness index was weighted on the mean daily rainfall, i.e. the slope of the evenness line. Significant correlations were also found by recalculating the indices over strategic short-term periods for maize, which indicates how the effectiveness of these indices may be increased by choosing appropriate periods for different crops. Finally, the different indices showed no correlation, indicating little redundancy and thus suggesting a profitable conjoint use of them.