Comparison of ground based observation of precipitation with TRMM satellite estimations in Fars Province

Precipitation which is a key parameter in agricultural and hydrometeorology studies has a large spatio-temporal variation. Hence, its small scale monitoring is quite difficult using ground-based data. Satellite images can provide fine resolution data of rainfall for different applications. However, the temporal and spatial verification should be evaluated and studied before using satellite data. In the present study, daily precipitation data from 12 synoptic and 37 rain gage stations of Fars province, Iran was used to evaluate level 3 B42 version 7 product of the Tropical Rainfall Measuring Mission (TRMM) satellite on a regular spatial scale (0.25° ×0.25°) for the period 1998-2011. The Pearson correlation coefficient, mean, median, variance, inter quintiles range (IQR), normalized root mean square error (NRMSE) and mean difference (MD) for daily, monthly, seasonal, yearly time scales were worked out and analyzed. In general, there was a significant correlation between TRMM and ground-based precipitation data over the most parts of the province for all time scale at 5% level such that the range of Pearson correlation for all synoptic (Except for Eghlid) and rain gage (Except for 6 stations) annual time scale was (0.8-0.95) and (0.64-0.93), respectively. The corresponding values for monthly time scale were (0.64-0.96) and (0.5-0.9). The highest and lowest correlation were observed in the monthly and daily time scales respectively. The range of NRMSE for annual, monthly and daily scales were (0.12 – 0.92), (0.21 – 0.94) and (0.79 – 1.91) respectively. In north western parts of the province, which receives more rainfall, positive mean difference values (the mean of observed precipitation was more than that of satellite precipitation) was observed for all time scales. Over eastern parts, in which the amount of precipitation is low, negative MD values were observed. The results of two sample t test indicated that there is no significant difference between means of observed and satellite precipitation for wet months at 5% significance level for 29 stations. Also, the results of F test indicated that the variance of the observed and satellite precipitation for wet months are the same at 5% significance level for 33 stations.