Hydrophobic Soil: Hydrophobic soil is soil that is resistant to wetting, meaning that it does not absorb water easily. This can occur when the soil contains a high percentage of organic matter or when it has a waxy or oily surface, which can make it difficult for water to penetrate the soil and be absorbed. When hydrophobic soil is present, it can increase runoff and contribute to stormwater production, as well as increase the severity and frequency of flash flooding.
Hydrophobic soils increase runoff: One way in which hydrophobic soil can increase runoff is by creating a surface layer that is resistant to infiltration. When it rains, the water will collect on the surface of the soil and run off into streams and rivers, rather than being absorbed and infiltrating the soil. This can lead to increased stormwater production, as the water is not being retained in the soil and is instead flowing into surface water bodies.
Increased flash flooding: Hydrophobic soil can also increase the severity and frequency of flash flooding. When soil is hydrophobic, it is less able to absorb water, which means that it is more prone to flooding. During heavy rainfall events, the water is more likely to run off the surface of the soil and collect in low-lying areas, causing flash flooding. This can be especially dangerous in urban areas where the increased runoff can cause flooding and damage to buildings and infrastructure.
Upcountry rains cause Flash Flooding in Kula Kai (coastal Kihei).
Here are some articles about the impacts of hydrophobic soil on runoff and flooding:
Shakesby, Richard & Doerr, Stefan & Walsh, R.P.D.. (2000). The Erosional Impact of Soil Hydrophobicity: Current Problems and Future Research Directions. Journal of Hydrology. 231-232. 178-191. 10.1016/S0022-1694(00)00193-1. https://www.researchgate.net/publication/223534820_The_Erosional_Impact_of_Soil_Hydrophobicity_Current_Problems_and_Future_Research_Directions
Soil water repellency and infiltration in coarse-textured soils of burned and unburned sagebrush ecosystems, https://www.fs.usda.gov/research/treesearch/32747
Land use change impacts on floods at the catchment scale: Challenges and opportunities for future research, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5575485/
- Surface Water in Hawaii, https://pubs.usgs.gov/fs/fs04503/htdocs/fs045-03.html