WISQQiH - Background


CONTENTS:

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1. The Humber Watershed
2. The Impact of Urbanization on Watershed Hydrology
3. Determining Land Cover via Remote Sensing
4. Determining Catchment Areas using Digital Elevation Models
5. The SCS Method of Runoff Modelling

1. The Humber Watershed

The Humber Watershed covers 908 sq-km and is home to 485 000 people. It is the largest of the 9 watersheds in the Metropolitan Toronto and Region Conservation Authority (MTRCA) jurisdiction. Groundwater originating in the Humber basin feeds more than 100 tributaries (MTRCA, 1995).

Within the Humber there are five distinct physiographic regions. These play a strong role, along with climate, in determining the functioning of the hydrologic cycle (and in determining habitat for flora and fauna). The five regions are (1) the Niagara Escarpment (2) the Oak Ridges Moraine (3) Peel and Iroquois Plains (4) Valley and Stream Corridors, and (5) Waterbodies (HWTFNHS 1996).

The Watershed is a dynamic system. The greatest changes to the Watershed since the last glaciation have occurred in the last 200 years, with human impacts.

Impacts in the Oak Ridges Moraine are of particular concern with regard to the hydrologic cycle. The Moraine, encompassing 27% of the area of the Watershed (HWTFNHS 1996), is comprised of coarse, well-drained soils. The coarse soils allow meltwater and rainfall to soak into the ground, feeding aquifers. These aquifers, in turn, discharge cold, and relatively clean, water, feeding the headwaters of the East Humber and Main Humber rivers, as well as their tributaries (HWTFNHS 1996). The aquifers also provide the principal water supply for many communities and rural residents. Fortunately, the Moraine is largely in a relatively undisturbed state. However, development in the Moraine threatens the long-term quality and quantity of local and regional water resources.

While the Oak Ridges Moraine is vital, urbanization in any part of the Watershed impacts the hydrology of the basin. The theoretical background with regard to the impact of urbanization on the Watershed is discussed in the next section.

Resources:

  • Madelyn Webb of the MTRCA at (416) 661-6600, ext.331.
  • Humber Watershed Task Force Natural Heritage Sub-Committee (HWTFNHS) 1996. Strategies for the Protection and Enhancement of the Natural Heritage System in the Humber River Watershed.
  • Metropolitan Toronto and Region Conservation Authority (MTRCA). 1995. The Humber Advocate (newsletter).
  • Ministry of Environment and Energy (MOEE). 1993. Water Management on a Watershed Basis: Implementing an Ecosystem Approach. Toronto, Ontario: Queen's Printer for Ontario.
  • Oak Ridges Moraine Technical Working Committee (ORMTWC). 1994. The Oak Ridges Moraine Area Strategy For the Greater Toronto Area - Draft for Public Discussion. Maple, Ontario: Queen's Printer for Ontario.

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    2. The Impact of Urbanization on Watershed Hydrology

    Water is a vital component of the natural environment, influenced by climate and land, and sustaining life. This is best understood in terms of the hydrologic cycle. The hydrologic cycle can also be seen as a system diagram.

    Urbanization impacts the hydrologic cycle in two key ways. Through (1) increasing population density, and (2) increasing building density. This can be represented graphically. The result is a more complicated hydrologic cycle. There are two major effects of urbanisation on water quantity, (1) increases in runoff volumes, and (2) increases in peak flow rates. Urbanization impacts water quality in that it results in increased loads of metals and organic contaminants.

    The impact of urbanization on watershed hydrology has been explored in a number of studies. Cook et al. (1985) completed a study of the Speedvale Experimental Basin (a 210 ha watershed on the outskirts of Guelph, Ontario). A relatively extensive hydrologic data base of precipitation, streamflow, soil moisture, and groundwater was assembled both for the pre-urbanization period (1966-74) and the period of development (1975-82). The study area was used for agricultural purposes in the first stage. In the second period, 155 ha was serviced for light industrial and commercial development, which involved the installation of a stormwater conveyance system. Urbanization resulted in an increase in impervious area from 1% to 33% of the basin.

    Cook et al. (1985) found that "...changes in land use coincided with changes in volumetric and time distribution aspects of hydrologic response. Changes in the response include:

    (i) an increase in the mean annual runoff coefficient by a factor of 1.5;
    (ii) an increase in the instantaneous discharge by a factor of almost 3.0;
    (iii) a change in the time of annual peak flow from occurring solely in the spring runoff period to occurring throughout the various seasons;
    (iv) a change in the seasonal pattern of monthly and storm event runoff coefficients with the greatest change observed in the summer and lesser changes observed in the other seasons;
    (v) a 3 fold reduction in unit hydrograph lag time; and
    (vi) a 3.5 fold increase in unit hydrograph peak flow".

    Boyd et al. (1993) completed a similar study, except that they contrasted pervious and impervious surfaces rather than urban and rural areas. They studied runoff from pervious versus impervious surfaces in 26 urban catchments in 12 countries. Rainfall and runoff depths were examined for 763 storm events. It was found that the effective impervious area remained constant for all storm sizes. This is significant in that it indicates that impervious surfaces will cause the vast majority of precipitation to runoff, with little other system response. Pervious runoff, by contrast, was determined by the depth of rain in the storm for rainfalls less than 50 mm, while rainfall intensity and antecedent wetness were also significant with regard to runoff from larger storms.

    Stephenson (1994) studied two topographically similar, and adjacent, catchments near Johannesburg, South Africa over a five year period. One of the catchments was suburban while the other was natural grassland. Surface runoff from the undeveloped and suburban catchments was 4% and 15% respectively. Evapotranspiration was 63% for both catchments. The study concluded that suburban development increased surface runoff volume by a factor of four.

    It has long been recognized that urban runoff contains a diverse range of pollutants, which can occur in concentrations that are harmful to the ecosystem. A profile of impacts was developed by the U.S. Environmental Protection Agency in 1992, which found that "between one-third and two-thirds of all designated use impairment for...streams was a result of agricultural and urban runoff" (Debo & Reese, 1995: 673).

    Although there is a dearth of hard data, it is generally acknowledged that urban runoff contains significant levels of pollution and is a major source of receiving water pollution. The Humber Watershed Task Force has recognized that industrial land use generally contributes relatively high concentrations of metals, while agricultural and residential land use tends to result in high organic concentrations in runoff (1996).

    Resources:

  • Boyd, M.J., Bufill, M.C. & Knee, R.M. 1993. Pervious and impervious runoff in urban catchments. Hydrological Sci. J. 38,6:463-78.
  • Chow, V.T., Maidment, D.R. & Mays, L.W. 1988. Applied Hydrology. New York, NY: McGraw-Hill Book Company.
  • Cook, D.J. & Dickinson, W.T. 1986. The impact of urbanization on the hydrologic response of a small Ontario watershed. Can. J. Civ. Eng. 13:620-30.
  • Debo, T.N. & Reese, A.J. 1995. Municipal Stormwater Management. Boca Raton, Florida: Lewis Publishers.
  • Hall, M.J. 1984. Urban Hydrology. Northern Ireland: Elsevier Applied Science Publishers.
  • Stephenson, D. 1994. Comparison of the water balances for an undeveloped and a suburban catchment. Hydrologic Sci. J. 39, 4:295-307.

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    3. Determining Land Cover via Remote Sensing

    As explained in the above discussion, both theory and empirical studies indicate that changes in land use resulting from urbanisation are linked to changes in resulting runoff volumes and rates.

    In order to track changes in land use over time, a data source for land use is required that describes land use in a fashion that is both sufficiently detailed and easily processed for analytical purposes. Multispectral remotely sensed images are an ideal data source for this application.

    Remote sensing is defined as the measurement, from a distance, of the spectral futures of the earth's surface (Mather 1987). Data collected over time can be used to determine changing land cover/use.

    Once a land use classification map is derived from the multi-spectral images, land uses can be associated with runoff production through some form of hydrological modelling.

    Resources:

  • Engman, E.T., & Gurney R.J. 1991. Remote Sensing in Hydrology. London, England: Chapman and Hall.
  • Mather, P.M. 1987. Computer Processing of Remotely Sensed Images. Chichester, England: John Wiley & Sons.

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    4. Determining Catchment Areas using Digital Elevation Models

    In order to associate the land use classifications derived from multispectral remote sensed images with the stream quality and quantity data taken at a point on the stream network, the catchment that drains the network to that point must be determined. By defining the catchment associated with a given stream gauge, two objectives are fulfilled: 1) The area of the catchment is known, allowing volumetric discharges to be converted to depth equivalent values for easy computation and comparison with precipitation. 2) The spatial region that drains the landscape to the stream quality/quantity station is explicitly identified, allowing the computation of descriptions of land cover that are directly associated with the stream station.

    Determining a catchment area from a raster digital elevation model (DEM) involves the invoking of the most basic hydrologic rule, namely that water flows downhill. A raster DEM conceptualises the landscape in terms of a large grid of adjacent squares that each have an associated value of altitude. Applying the downhill flow rule to this model of the landscape results in water from a square (pixel) will flow to the adjacent square that has the lowest elevation. Using this principle, a drainage direction map is produced that indicates in which direction any given pixel will drain. By specifying the location of the stream monitoring station, the associated catchment can be calculated from the drainage direction map, by working uphill from the location, finding every pixel that eventually drains to the location.

    The above description is a somewhat simplified expression of what is involved algorithmically. In a practical sense, these issues are simplified for the user of GRASS as well, as the procedure is easily implemented through the r.watershed (drainage direction generation) and r.water.outlet (catchment for a stream outlet) programs.

    Resources:

  • Band, L.E. 1986. Topographic Partition of Watersheds with Digital Elevation Models. Water Resources Research. 22:15-24.

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    5. The SCS Method of Runoff Modelling

    The Soil Conservation Service (1972) developed a method for abstracting runoff from rainfall based on the land use and soil type characteristics of the catchment. Based on these two factors, an appropriate SCS curve number (CN) can be selected from a table of values (determined through empirical analysis of several catchments). Also, a further table provides CNs for urbanized areas. Once a CN is chosen, resulting runoff can be determined from precipitation using an equation or diagrammatic form of the SCS model. Using this procedure, simulated runoffs are calculated and compared to actual runoffs. The model is calibrated by modifying the CN value until the model predicts runoff accurately.

    Resources:

  • Akan, A.O. 1993. Urban Stormwater Hydrology: A guide to engineering calculations. Lancaster, Pennsylvania: The Technomic Publishing Co. Inc.
  • Chow, V.T., Maidment, D.R. & Mays, L.W. 1988. Op. cit.
  • Engman, E.T., & Gurney R.J. 1991. Op. cit.
  • Stephenson, D. 1981. Stormwater Hydrology and Drainage. Amsterdam, The Netherlands: Elsevier Scientific Publishing Co.

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