Monthly average surface runoff refers to the quantity of water that flows across the land surface and into water bodies on a monthly timescale.
In more detail:
Surface runoff is the water that travels overland after precipitation falls. This includes water that does not infiltrate into the soil and flows over the land before reaching streams, rivers, lakes or oceans.
The monthly average surface runoff is calculated by taking the total surface runoff for a given month and dividing by the number of days in that month. This gives an understanding of the typical or average daily surface runoff behavior over longer monthly time periods.
The metric aims to capture seasonality and variability in surface runoff patterns over the course of the year. Some months, often those aligned with wet seasons or high precipitation, will exhibit higher average surface runoff.
Monthly values are more useful for understanding surface runoff behavior than daily data because high day-to-day variability can obscure longer term patterns. Averaging helps characterize the central tendency.
Factors that influence the monthly average surface runoff include soil moisture conditions, recent precipitation patterns, vegetation/land use, topography, and the prevalence of rain vs. snow.
The units are typically expressed as depth of water over the land area being considered, e.g. mm/month or equivalent water depth averaged per day in the month.
Monitoring monthly surface runoff gives insight into regional hydrology and how it changes across wet vs. dry seasons. It also helps inform water resource management and flood or drought risk analysis.
