I wonder how many HRU areas should I do in my catchment area. The catchment area of 30,000 ha is divided into 31 sub-basins. Upland area with a slight slope. 14 landuses. The 3 landuses are separated by the split landuses function into 6 additional landuses. Total landuses there are 20. 32 types of soils. 5 groups of terrain slopes. I used filter by landuse, soil, slope (thresholds) 15% landuses, 15% soils, 10% slope. I received 1,000 HRU areas. Is it correct? Should I accept fewer HRU areas?
It really depends on the spatial variability of 1) the soil type, and 2) land cover. I find that its best to experiment with the HRU be increasing/decreasing its number and monitoring the effect that will have on runoff at a certain point. Sometimes you get to a point where increasing the HRU does not have an effect on results.
It isn't very easy, but I would like to suggest these items:
a) The initial percentage of each landuse types
b) The percentage of each landuse types after creating HRUs
It would be best if you tried to keep "a" and "b" close together, especially if you have croplands with irrigation. Of course, soil and slope are important too, but, first of all, think about landuses.
I am not familiar with SWAT model, and I assume HRU is hydrologic response units. In the past, we used third and fourth order (Strahler) for some analyses, as they were typical headwater perennial streams in the mountains and piedmont in the SE USA National Forests. I have since found that LiDAR adds about 2 orders to those using the 1:24,000 topographic map scale DEMs. You might be interested from my Researchgate in stream types and management interpretations, and the EIS I uploaded from the loblolly pine removal and restoration project that used drainage response units to address issues of sediment and water yield from harvest effects. It may just depend on how many units you prefer to analyze to address your management goals.
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