Climatology

The Climatology object in GeoSWMM describes the following climate-related variables used for computing runoff and snowmelt:

·       Temperature

·       Evaporation

·       Wind Speed

·       Snowmelt

·       Areal Depletion

·       Climate Adjustments

Temperature Air temperature data are used when simulating snowfall and snowmelt processes during runoff calculations. They can also be used to compute daily evaporation rates. If these processes are not being simulated then temperature data are not required. Air temperature data can be supplied to GeoSWMM from one of the following sources: • A user-defined time series of point values (values at intermediate times are interpolated) • An external climate file containing daily minimum and maximum values (GeoSWMM fits a sinusoidal curve through these values depending on the day of the year) For user-defined time series, temperatures are in degrees F for US units and degrees C for metric units. The external climate file can also be used to directly supply evaporation and wind speed as well.

Evaporation Evaporation can occur for standing water on subcatchment surfaces, for subsurface water in groundwater aquifers, and water held in storage units. Evaporation rates can be stated as:
• A single constant value • A set of monthly average values
• A user-defined time series of daily values • values computed from the daily temperatures contained in an external climate file • daily values read directly from an external climate file. If rates are read directly from a climate file, then a set of monthly pan coefficients should also be supplied to convert the pan evaporation data to free water-surface values. An option is also available to allow evaporation only during periods with no precipitation.

Wind Speed Wind speed is an optional climatic variable that is only used for snowmelt calculations. GeoSWMM can use either a set of monthly average speeds or wind speed data contained in the same climate file used for daily minimum/maximum temperatures.

Snowmelt Snowmelt parameters are climatic variables that apply across the entire study area when simulating snowfall and snowmelt. They include: • The air temperature at which precipitation falls as snow • Heat exchange properties of the snow surface • Study area elevation, latitude, and longitude correction

Areal Depletion Areal depletion refers to the tendency of accumulated snow to melt non-uniformly over the surface of a subcatchment. As the melting process proceeds, the area covered by snow gets reduced. This behavior is described by an Areal Depletion Curve that plots the fraction of the total area that remains snow covered against the ratio of the actual snow depth to the depth at which there is 100% snow cover. A typical ADC for a natural area is shown in Fig. 50. Two such curves can be supplied to GeoSWMM, one for impervious areas and another for pervious areas.

Fig.50: Areal Depletion for a natural area (Source: User Manual for EPA-SWMM 5.1)

Adjustments Adjustments are optional modifications applied to the temperature, evaporation rate, and rainfall intensity that GeoSWMM would otherwise use at each time step of a simulation. Separate sets of adjustments that vary periodically by month of the year can be assigned to these variables. They provide a simple way to examine the effects of future climate change without having to modify the original climatic time series. Similarly manner, a set of monthly adjustments can be applied to the hydraulic conductivity used in computing rainfall infiltration on pervious land surfaces and exfiltration from storage nodes and conduits. These can reflect the increase of hydraulic conductivity with increasing temperature or the effect that seasonal changes in land surface conditions, such as frozen ground, can have on infiltration capacity.