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Title: Precipitation data considerations for evaluating subdaily changes in rainless periods due to climate change

Author
item Bonta, James - Jim

Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/27/2012
Publication Date: 5/28/2013
Citation: Bonta, J.V. 2013. Precipitation data considerations for evaluating subdaily changes in rainless periods due to climate change. Journal of Soil and Water Conservation. 68(3)238-253.

Interpretive Summary: Often subdaily precipitation data are lacking for modeling watersheds for conservation investigations of water quality, and the data must be synthesized by computer modeling. Furthermore, precipitation patterns may be undergoing change due to changes in climate. One important element for synthesizing precipitation is the dry times between storms (TBS). Two variables characterize TBS, minimum TBS (MTBS) and average TBS (ATBS) – both vary monthly. Detecting changes in these parameters due to climate are important, but there are issues with depth resolution of precipitation data that must be investigated first to arrive at valid conclusions. Precipitation data having 1-min temporal and 0.25-mm depth resolutions (high resolution data) from the UDSA-Agricultural Research Service, North Appalachian Experimental Watershed (NAEW) at Coshocton, Ohio were used to determine the spatial variability and consistency of estimates over the 425-ha NAEW, and the impacts of “coarsening” the depth resolution of data to 2.54 mm. Additionally, 5-yr periods over the recent 60+ years were evaluated for trends due to climate change. Small spatial scale precipitation data showed consistent MTBS and ATBS long-term values for each month with high resolution data. 60-yr trends for small spatial scale high resolution data are consistent. A coarser data resolution can give generally consistent trend indications for MTBS, but not for ATBS. There is a weak indication that the fall months may be affected by significant shortening of ATBS at the NAEW and in NE Ohio, but more data are needed. The coarse resolution data may not be adequate to characterize TBS and investigate changes due to climate change unless more data manipulation is applied, such as identifying and excluding “dry” times when precipitation may have occurred. Mixed resolution data cannot be used to detect climate changes of dry periods. Much more high quality data are needed to arrive at firm conclusions regarding climate change effects on TBS.

Technical Abstract: Quantifying magnitudes and frequencies of rainless times between storms (TBS), or storm occurrence, is required for generating continuous sequences of precipitation for modeling inputs to small watershed models for conservation studies. Two parameters characterize TBS, minimum TBS (MTBS) and average TBS (ATBS) – both vary monthly and are calculated by assuming TBS follows an exponential distribution. Detecting changes in these parameters due to climate is important for precipitation and climate studies at short time and small spatial scales, but there are issues with depth resolution of precipitation data that must first be investigated. Precipitation data having 1-min temporal and 0.25-mm depth resolutions from the UDSA-Agricultural Research Service, North Appalachian Experimental Watershed (NAEW) at Coshocton, Ohio were used to determine the spatial variability and consistency of estimates over the 425-ha facility, and the impacts of “coarsening” the depth resolution of data 10 times to 2.54 mm. The magnitudes and trends of changes in MTBS and ATBS are determined for 3 rain gauges over the NAEW, and extended to other gauges in the Ohio region. Approximately 5800 parameter pairs were computed for different data configurations. Small spatial scale, NAEW high resolution precipitation data, shows consistent MTBS and ATBS long-term values for each month, but more variability in estimates for MTBS than ATBS. Climate-change trends for MTBS and ATBS are not apparent and generally consistent for different NAEW gauges, except for Nov when all gauges were consistently significant for ATBS. There is a weak indication that the fall months may be affected by significant shortening of ATBS at the NAEW and in NE Ohio due to climate change, but more data are needed. An imposed coarser NAEW data resolution results in consistent no-trend indications for MTBS, but not for ATBS. The coarse resolution data may not be adequate to characterize TBS for investigating changes due to climate change unless more data preparation is performed, such as identifying and excluding “dry” times when precipitation may have occurred. Precipitation records with mixed resolution data cannot be used to detect climate changes of dry periods. More data and data preparation are needed to arrive at firm conclusions regarding climate change effects on TBS in Ohio.