Application of geostatistical methods in determination of depth-area-duration rainfall curves (Lorestan province)

Introduction
One of the main goals of spatial analysis of precipitation at area is to reach the standard project storm (SPS) for that area, through which can be reached the standard project flood (SPF). This analysis includes the characteristics of rainfall depth at a certain area and for a specific duration. Relation between depth and rainfall area which called depth-area-duration (DAD), is shown usually by set of curves that each shows different duration of rainfall. Using these curves, a reduction factor is determined for specific area and is applied to adjust the average point rainfall related to frequency of this project. The present study carried out under topic of investigation and map of depth- area- duration in Lorestan, in an area over 28559/5 Km2 in west part of country.
Materials and Methods
Weak coefficient of correlation is shown meaningfulness of the relation between rainfall and altitude in different time base. this is resulted from different reasons such as extension of area, lack of transmittal and number of suitable weather stations and different extension and tension of mountain than rain flaw. Considering above points cause those other methods of drawing precipitation maps include interpolation or geostatistical methods including, spline, IDW, kriging and Co-kriging were used.
Results and Discussion
Weak coefficient of correlation is shown meaningfulness of the relation between rainfall and altitude in different time base. this is resulted from different reasons such as extension of area, lack of transmittal and number of suitable weather stations and different extension and tension of mountain than rain flaw. Considering above points cause those other methods of drawing precipitation maps include interpolation or geostatistical methods including, spline, IDW, kriging and Co-kriging were used.
Conclusion
The results show that to preparing precipitation maps of selecting storms, simple co- kriging (SCK) is a suitable method to calculate the amount of rainfall of selecting storms in lorestan province. So the above way is used for preparing precipitation maps. Resulted from investigation of surface reduction factor of rain fall shows that in time duration 12 and 48 hrs with the increase of each 5000 surface reduction factor reduces for 0.1 in 24 hrs duration within 18000 Km2. This coefficient has a slow decreasing trend and then that is similar to 12 and 48 hrs rainfalls. Assessing the daily rainfall statistics of some of rain gauge stations in somewhere of the province by the Meteorological Organization and the Ministry of Energy, shows that sometimes there is a significant difference between the perception rates recorded by these organizations. Therefore, in order to eliminate the existing defects, it is suggested that the stations occupied by these organizations and their monitoring status be periodically evaluated by the experts of the relevant organizations and possible defects be prevented.