Overview of Runoff
- Runoff is often defined as the portion of rain, snowmelt, and irrigation water that moves toward the stream channel rather than infiltrating the soil.
- For some purposes, however, runoff also includes the subsurface water known as interflow which also quickly moves toward the stream channel.
- Runoff is the most important component of flood prediction.
- The three soil water processes are:
- 1) the entry or infiltration of water into the soil
- 2) the transmission of water within the soil
- 3) the storage of water as soil water
- Important runoff terms include:
- Basin – the area that drains to a single outlet point
- Baseflow – the long-term supply that keeps water flowing in streams
- Infiltration – the downward movement of water through the soil surface
- Percolation – the movement of water within the soil profile
- Surface runoff – the movement of water across the soil surface to the stream channel
- Interflow – the relatively rapid movement of water beneath the soil surface to the stream channel
- Runoff – sometimes this is just the surface runoff, but it can refer to a combination of surface runoff and interflow.
- Infiltration rate – the amount of water able to enter the soil in a specified time
- Infiltration capacity – the upper limit of the infiltration rate
- Surface runoff equals the rainfall or snowmelt rate minus the infiltration capacity.
Paths to Runoff
- There are two types of surface runoff:
- Infiltration excess overland flow (Hortonian flow)
- Saturation excess overland flow
- Infiltration excess overland flow occurs when the rainfall rate exceeds the infiltration capacity and can occur even in dry soil conditions.
- Saturation excess overland flow occurs when the soil is saturated and no more rain or snowmelt can infiltrate.
- Interflow is the subsurface movement of water to the stream and can be a significant component of runoff.
- Transmissivity feedback occurs where macropores are activated by interflow.
- Macropores are natural voids and pipes in the soil that increase interflow and are formed by biological and chemical activity.
- Enhanced interflow may occur along a soil-bedrock interface, especially if the bedrock is sloped.
- A fragipan is an area of relatively low permeable soil and rock that can block the downward percolation of water and increase horizontal water movement.
- Groundwater ridging occurs when surface rain or snowmelt reaches the water table more easily in a given area causing the water table to bulge upward—this can increase interflow.
- Pre-event water is water that exists in the soil profile when a storm begins and gets displaced into the stream by newly infiltrated water. It can be a very important contributor to runoff as interflow.
Basin Properties
- With uniform basin characteristics and coverage of rainfall or snowmelt, larger basins result in a larger volume of runoff.
- Uniform coverage is unusual, so runoff is determined by the part of the basin that receives the rainfall; this is known as the contributing area.
- For basins of equal area but different shapes, a long narrow basin will result in a hydrograph that has a lower peak flow and is spread out over a longer time when compared with a “rounder” basin.
- When compared to a meandering stream, a straight stream will result in a stream response that is faster and has a higher peak flow.
- When compared to a relatively flat basin, a steeply sloped basin will result in a stream response that is faster and has a higher peak flow.
- When compared to a stream channel with rough surfaces (lots of rocks and vegetation), a smooth stream channel will result in a stream response that is faster and has a higher peak flow.
- Stream density is the length of all the channels in a basin divided by the basin area.
- High stream density indicates a basin with numerous tributaries and faster, more efficient runoff.
- Urbanization results in increases in both the amount and the speed of runoff.
Soil Properties
- Soil texture is important for anticipating infiltration and storage of water and is determined by the diameter of the soil particles.
- Clay has the smallest soil particle diameter, followed by silt, and then sand with the largest particles.
- Sandy soil allows water to infiltrate and drain more quickly than clay soil.
- Clay soil can hold a larger volume of water than sandy soil, but runoff during intense rainfall is more likely with clay due to its lower infiltration rate. Clay also stays saturated longer than sand.
- Depth to impermeable layers such as bedrock or a fragipan can influence how much water the soil layer can hold and how quickly it will saturate.
- Shallow soils typically saturate more quickly.
- Surface characteristics are very important for determining the potential for rapid surface runoff.
- Urbanization – impermeable and compacted surfaces increase the amount and speed of runoff
- Deforestation and forest fires – removal of vegetation and burning of the soil can increase the amount and speed of runoff as well as the sediment loading in the runoff, especially in sloped areas due to the loss of vegetation and the decrease in infiltration
Runoff Modeling Concepts
- The Rational Method is a simple runoff model that takes into account land use characteristics for small basins using a land-use runoff coefficient, or “C” value.
- “C” values vary from near zero in rural or forested areas to almost 1.0 in urban areas and parking lots.
- Lumped models are relatively simple in that the basin is treated as a single unit
- Basin-averaged hydrologic and meteorological inputs are used
- A semi-distributed or pseudo-distributed model breaks the basin down into a collection of smaller sub-basins than the lumped-model approach
- Distributed models represent details of the hydrologic and meteorological characteristics by representing basins with a grid.
- The distributed modeling approach is:
- Much better at representing variability in hydrologic response within the basin
- More realistic for flash flood modeling
- More computationally intensive than lumped modeling
National Weather Service River Forecast System (NWSRFS)
The National Weather Service River Forecast System components include:
- Calibration - uses historical data
- Operational Forecast System (OFS) - used to adjust model parameters and
input for both long- and short-term hydrologic forecasts
- Interactive Forecast System - forecaster adjusts graphical products and
forecasts
- Ensemble Streamflow Prediction (ESP) - preparation of long-term forecasts
and probabilistic guidance
- The six sub-components of NWSRFS include:
- Rain and snow operations
- Snow modeling
- Hydrologic modeling
- Unit hydrograph operations
- Adjustment to match simulated with observed streamflow
- Routing operations
- The Sacramento Soil Moisture Accounting (SACSMA) model:
- Is a spatially lumped, continuous runoff model used by many NWS offices within NWSRFS
- Accounts for day-to-day soil moisture changes
- Is ideally suited for large basins
- Divides each basin into a surface boundary, an upper soil zone, and a lower soil zone
- Accounts for direct runoff from impermeable surfaces
- Accounts for infiltration, percolation, interflow, and long-term storage
- Accounts for evapotranspiration
- A computed hydrograph from SACSMA represents the different components of the flow from rapid direct runoff to long-term baseflow.
- The Antecedent Precipitation Index (API) model is an event-driven simple statistical model that is used by some offices within NWSRFS.
- It is based on basin characteristics, time of year, storm duration, and rainfall amount.
- Because it is event-driven, it is not useful for water supply guidance.
- A continuous API model is a more complex form of the API model that is not event-driven, and therefore can be used for water supply guidance.
- It uses information about soil moisture, surface moisture, incremental surface runoff, and groundwater storage.