Submitted to: Journal of Theoretical and Applied Climatology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/30/1999
Publication Date: N/A
Interpretive Summary: Rainfall can be extremely variable across small distances and it has been observed that rainfall can vary from field to field. It is critical for decision-making by farmers, hydrologists, and environmental planners that we understand the variation within small watersheds. Most of the previous research has been conducted at large scales of over 100 km**2 while many of the decisions need to be made at the scale of less 10 km**2. This study was undertaken in the Walnut Creek watershed near Ames, Iowa, to evaluate the variation among 23 raingages placed throughout the watershed on individual precipitation events and monthly and yearly totals. Although there was a large variation among raingages for a precipitation event there was no consistent variation among raingages. Rainfall varied among raingages, however, there was no differences among raingages when the monthly and yearly totals were calculated. In the Midwest, there may be a large variation for an individual storm, however, on a yearly total there was no differences among the raingages. For planning purposes, the differences among raingages can be considered to be a random occurrence. Environmental quality modeling can be improved by understanding the small changes in rainfall within a watershed, however, this study indicates that direct measurement will be necessary to overcome the random variation. For decision-makers, e.g., farmers and consultants, it may be necessary to have an on-site raingage to overcome the problem of variation within small areas. This will increase the confidence in understanding the role of rain in real-time management decisions.
Technical Abstract: Rainfall amounts and the distribution across the landscape are critical to decision-making and evaluation of hydrological models. Spatial variation in rainfall has been observed through anecdotal evidence and limited studies; however, there is little quantitative evidence that can be used to assess rainfall variation within a watershed on a daily, monthly, or yearly temporal scale. This study was conducted to quantify the spatial variation within a watershed in central Iowa and to determine if there were consistent differences among raingages. The study was conducted within Walnut Creek watershed located south of Ames, Iowa, on the Des Moines landform region. The topography of this 5130 ha watershed is characterized by gently rolling fields with a narrow area of steeper land along the stream in the lower part of the watershed. Twenty-two tipping bucket rain gages were placed throughout the watershed and rainfall was recorded as 5 minute totals and then aggregated into daily totals. Accumulation of errors of the 5 minute values into the daily totals were considered to be random. There was a large coefficient of variation in the average daily totals; however, there was no consistent pattern of variation among raingages, and coefficient of variation decreased with amount of rain. Each rain gage had an equal chance of receiving the lowest or highest rainfall total for any given storm event. When the daily average was computed over the year, there were no differences among rain gages. Monthly and yearly totals showed a decreased coefficient of variation compared to daily totals. There was no consistent pattern of spottiness within the watershed and if daily rainfall amounts are required for a decision, then direct measurements may be required.