Question 1 of 4: Ontario Hydrologic Abstractions and Watershed Characteristics

Question 1: Ontario Hydrologic Abstractions and Watershed Characteristics (33.3 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given. A qualitative/short-answer question on Southern-Ontario hydrologic norms, the hydrologic-abstraction processes, and the physical controls on a storm hydrograph — no numeric data is supplied.

Find. 1.1(a–h) eight short factual/definitional answers; 1.2(a–d) high/moderate/low ratings with controlling factors for four abstractions across the Southern Ontario Great Lakes basin; 1.3 the effect of four watershed characteristics on hydrograph peak, volume and duration.

Approach. Answer each sub-part directly from standard Ontario climate normals and the hydrologic-cycle/watershed-response theory in the reference texts; no calculation is required for this question.

1.1(a) – Long-term annual average precipitation in Ontario. Roughly 800–1,000 mm/yr over most of Southern Ontario (e.g. Toronto ≈800 mm, London/Windsor snowbelt areas approaching or exceeding 1,000 mm because of Great-Lakes-effect snow); Northern Ontario is generally lower, around 600–800 mm/yr.

1.1(b) – Percentage of precipitation converted to evapotranspiration. On a long-term (annual) basis, roughly 50–60% of Ontario's precipitation returns to the atmosphere as evapotranspiration, with the remaining 40–50% appearing as surface runoff and groundwater recharge that eventually reaches stream flow.

1.1(c) – Months of peak evapotranspiration. Most evapotranspiration occurs during the warm growing season, essentially May through September, with the peak in June–August when solar radiation, air temperature and active vegetation cover are all at a maximum.

1.1(d) – Actual vs. potential evapotranspiration. Potential evapotranspiration (PET) is the evapotranspiration that would occur from a well-watered reference surface under the prevailing atmospheric demand (radiation, temperature, humidity, wind) — it is a climate-driven upper limit, independent of soil moisture. Actual evapotranspiration (AET) is what really leaves the surface given the water that is actually available; AET ≤ PET always, with AET = PET only when soil moisture is non-limiting (e.g. shortly after rain or irrigation), and AET falling progressively below PET as the soil dries out.

1.1(e) – Recording vs. non-recording rain gauges. A non-recording (standard/manual) gauge gives only the total accumulated depth over the observation interval (typically 24 h) — it must be read manually and yields no information on when, how hard, or for how long the rain fell within that interval. A recording gauge (tipping-bucket, weighing or float type) continuously logs depth against time, producing a mass curve/hyetograph that captures intensity, duration and the time distribution of the storm — the data needed for IDF analysis, unit-hydrograph development or infiltration-capacity studies.

1.1(f) – Three components of the interception process. (1) Interception storage — the depth of water held on leaf, stem and canopy surfaces up to the vegetation's storage capacity; (2) throughfall — rain that passes directly through gaps in the canopy or drips off leaves to the ground once storage capacity is exceeded; (3) stemflow — intercepted water that runs down stems/trunks to the ground surface (with evaporation of the intercepted water back to the atmosphere during and after the storm being the associated interception loss).

1.1(g) – Two major components of stream flow. Base flow (the sustained, slowly-varying contribution from groundwater discharge between storm events) and direct runoff/storm flow (the quick-responding surface and shallow-interflow contribution generated by a specific rainfall event).

1.1(h) – Storage-indication vs. Muskingum routing. The storage-indication (modified Puls) method routes a flood through a reservoir/pond, where storage is a unique function of outflow (or water-surface elevation) ALONE, S = f(O), because a reservoir has negligible flow velocity/wedge effects; it solves continuity ($I - O = dS/dt$) with a single indication curve $2S/\Delta t + O$ vs. $O$. The Muskingum method routes a flood through a river/channel REACH, where storage depends on BOTH inflow and outflow, $S = K[xI+(1-x)O]$, representing the combined prism-plus-wedge storage of a moving flood wave; the weighting factor $x$ (0–0.5) reflects how much the wedge storage is controlled by inflow versus outflow, and the method reproduces both the translation and the attenuation of the wave along the reach.

1.2 – Spatial/temporal variation of hydrologic abstractions, Southern Ontario Great Lakes basin.

(a) Infiltration. HIGH on well-drained sandy/loam soils (e.g. the Norfolk sand plain) and in summer, when soils are unfrozen, unsaturated and macropores/root channels are active; LOW on clay-till soils, urbanized/paved surfaces, and in winter/early spring when soils are frozen or already saturated. Controlling factors: soil texture and structure, land use (row-crop agriculture vs. impervious urban cover), antecedent soil moisture, frost depth, vegetation/root density and surface slope.

(b) Evapotranspiration. HIGH June–August (peak radiation, temperature and active crop/forest canopy); LOW November–March (snow cover, dormant vegetation, low radiation, frozen soil). Spatially higher over forested and actively-cropped watersheds with full canopy cover than over urban/impervious land, where interception and transpiring leaf area are reduced. Controlling factors: net radiation, air temperature, wind, humidity (moderated by Great-Lakes proximity), vegetation type/cover fraction and soil-moisture availability.

(c) Interception. HIGH in dense forest cover, especially coniferous stands that intercept year-round; MODERATE in agricultural fields during the growing season; LOW in urban areas and in deciduous forest during the leaf-off winter period. Controlling factors: vegetation type, canopy density/leaf area index, season (leaf-on vs. leaf-off), and storm size — interception removes a much larger fraction of small, low-intensity storms than of large storms, where canopy storage saturates quickly.

(d) Depression storage. HIGH on flat agricultural land with tillage micro-relief and in urban areas with poorly-graded pavement/parking lots; LOW on steep or well-graded, smooth terrain. Controlling factors: land slope, surface roughness/micro-topography (tillage practice, paving condition) and soil type (which governs how quickly ponded depressions drain by infiltration).

1.3 – Effect of four watershed characteristics on the storm hydrograph.

Drainage area. A larger contributing area intercepts more of the storm and delivers a larger runoff VOLUME and a larger PEAK (more area contributing flow to the outlet simultaneously); it also lengthens the time base/DURATION of the hydrograph because water must travel farther, on average, to reach the outlet.

Slope. A steeper watershed shortens the time of concentration, so runoff arrives at the outlet faster and more synchronously — this sharpens and raises the PEAK and shortens the hydrograph's rising limb and overall DURATION, while total VOLUME (governed by rainfall and losses, not slope) is essentially unchanged.

Shape. A compact, roughly circular watershed concentrates flow paths of similar length, producing a sharp, high PEAK over a short DURATION; an elongated (fan- or pear-shaped) watershed spreads flow-path lengths over a wider range, producing a lower, more attenuated peak spread over a longer duration, for the same rainfall and area.

Land use / imperviousness. Urbanization (increased impervious cover, storm sewers) reduces infiltration, interception and depression-storage losses, raising the runoff coefficient — this increases both PEAK and VOLUME for a given storm, and the smooth, fast conveyance of storm sewers shortens the time of concentration, so the hydrograph rises and falls over a much shorter DURATION than the equivalent natural/agricultural watershed.