Dear researchers, I was trying to estimate and fix the volume of runoff collecting tank for small scale plots (36.5m^2) for two different land uses. I already know the runoff coefficient for each land uses. the question here is should I conduct rainfall frequency analysis or simply take rainfall intensity from the IDF curve?. If I have to use the IDF curve, then which return period should I use?
The runoff coefficient is typically for an annual period and seldom associated with individual events of varying magnitudes, durations and intensities. Conducting research in which collecting runoff, one would want to make sure there is ample capacity to store the amount from an extreme event. The runoff coefficient is apt to increase in an extreme event as compared to annual rate. Some land use practices can disturb soils enough so fine particles can seal off macropores, or hot severe wildfires temporarily make soils non-wettable. Soils with hard pans or extensive compaction could have higher rates. Then since you may want to drain tank during a dry period, you may want to consider the maximum or 100 year rainfall expected in 3 days. Consider the maximum runoff coefficient expected during dormant season and high incipient soil moisture. If snow is possible, you might need to add a factor for stored snow that would melt during a warm rain on snow event. Most runoff factors are calculated at catchment to watershed or basin scales. You may note that slow moving rain cells could potentially cover a plot with extreme rain intensity for extended periods, while this same cell is unlikely to be large enough to cover watershed or basin for an extended period. For these reasons, and knowing how important extreme events might be to this research, I would be extra conservative, and possibly install a temporary diversion into a 55 gallon or other suitable drum if the tank needs to be drained during an event, to avoid data loss.
You might be interested in my Hazel Pistol small runoff plot study in my researchgate papers, from work when I was in Oregon, addressing concerns associated with clearcutting and severe burning from residual fuels.
Another approach could be designed similar to tipping bucket rain gauge, only oversize the buckets to measure runoff from plot. It would take some instrumentation, but it could be designed to tip after each liter as an example, instead of storing the water. It would have to be designed and calibrated. Each tip of the bucket is recorded digitally or on a strip chart. Best to measure or record rainfall at each plot so you can adjust for plot comparison differences. For research, using standard size gauges and installation is best, but you can make them if funding is real tight, or use the inexpensive plastic wedge rain gauges if nothing else. If I have my numbers right, 1 mm runoff from your 36.5 sq meter plot would be 36.5 liters runoff.
I sincerely thank you for your valuable and helpful information that you provided to my question. as you said, the runoff coefficient is the typical factor depends on different conditions. the plots are gravel and cobble surfaces and I have got RC (runoff coefficient) from different kinds of literature and from her I should use the maximum Rc Value. I tipping bucket rain gauge will be installed and event-based rainfall-runoff responses will be recorded every 5 or 10 minutes. the storage tank that I wanted to use is 200L barrel but obviously not enough as you explained it before. so should I use multi-storage (barrel)?
3 blocks consisting of their own 4 plots (one plot is control point just for comparison) considering slope as a factor; Block 1 has 3 (1*3) plots having a 2% slope, and block 2 has also 3 plots having 5% and 7% slope. how to assign these blocks randomly? should either I use plots having different slopes in one block or plots with the same slope in one block only the arrangement of blocks to be considered?. T
hank you for you are assisting me with helpful information!