User’s Question:

I’m currently using the online Water Balance Model to develop a pre-to-post development water balance for a site in Ontario. I’m struggling to find agreement between the infiltration values from the online model and the ‘Thornthwaite and Mather’ (T&M) method of estimating infiltration.

The infiltration value using the online model exceeds 82% of the precipitation, whereas the T&M method gives a value of around 13% infiltration. The soil modeled in both cases is silty clay.

Obviously the two methods are very different in estimating infiltration, but I was not expecting such a large difference in values. Would you please provide me with guidance on how I could improve the model that I am working on?

Jim’s Response:

“The original work by ‘Thornthwaite and Mather’ related to the estimation of Potential Evapotranspiration. This is total Potential but not the Actual amount. It is the an estimate of the amount that would occur if there were sufficient water available to supply both the evaporation and the infiltration plus plant uptake of infiltrated water for transpiration.”

“Keep in mind that there is evaporation from the ground after infiltration and there is some losses to deeper penetration of water beyond the depth of the plant roots. As such this methodology has little to do with estimation of actual infiltration rates.

“I cannot help but think that you are trying to compare two different processes and the results of calculations done by different methods, even though some of the words describing them may be the same. I am also aware that there are software packages that use the work of ‘Thornthwaite and Mather’ to estimate a ‘water balance’; however, I have not examined them in detail except to note that they generally utilize monthly climate data.”

“The Water Balance Model utilizes hourly climate data and the calculation engine has been thoroughly tested over the past thirty years. We have confidence in the results of the calculation engine, provided user input data is reasonable. In my experience, one of the least understood components of the hydrologic calculations is the soil and its condition. The second misunderstanding that occurs is the use of infiltration rates used in the WBM. These refer to the water being discharged from the bottom of volume control facilities, not the surface infiltration rate used in the hydrologic calculations.”

About the Thornthwaite and Mather Method:

Chapter 3 in the Province of Ontario’s 2003 Stormwater Management Planning and Design Manual provides this description of the ‘Thornthwaite and Mather’ method:

3.2.2 Water Balance Methods: In cases in which the available data cannot support more sophisticated approaches, water balance methods are more appropriate for predicting the changes to the hydrologic cycle that may result  from urban development. They can be used to determine amounts of water that should be infiltrated to compensate for reductions caused by large paved areas or changes to vegetation. The water balance method developed by Thornthwaite and Mather (1957) determines the potential and actual amounts of evapotranspiration and water surplus (or excess of precipitation over evapotranspiration). Infiltration factors are used to determine the fraction of water surplus that infiltrates into the ground and the fraction that runs off to nearby streams. Thornthwaite and Mather’s method requires monthly or daily precipitation, monthly or daily temperature, latitude of the site, vegetation type, soil type, and a series of tables. The tables define a heat index, potential evapotranspiration, water holding capacity, and soil moisture retention. Snowfall, and alternating wet and dry cycles are included. Soil water holding capacity is dependent upon the soil type, soil structure and the type of vegetation growing on it. The Thornthwaite and Mather water balance method assumes mature vegetation and does not account for growing seasons where evapotranspiration would be less for immature vegetation.