Question 3 of 7: The Natural Hydrologic Cycle

Problem 3: The Natural Hydrologic Cycle (20 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.

(i) Hydrologic Cycle Schematic and Four Key Interactions

Four key components and their main interactions: (1) Precipitation delivers atmospheric moisture to the land and ocean surface, initiating every other pathway. (2) Evapotranspiration (evaporation from open water/soil plus transpiration from vegetation) returns moisture to the atmosphere, closing the loop and supplying the moisture that later falls as precipitation elsewhere — it is also the main way the cycle removes water from a watershed between storms. (3) Infiltration and subsurface (groundwater) flow moves precipitation that does not run off into the soil, recharging the water table; groundwater then flows slowly (weeks to centuries) toward streams and the ocean, sustaining baseflow in rivers during dry periods. (4) Surface runoff is the fraction of precipitation that neither infiltrates nor evaporates, flowing overland and through channels to streams, rivers and ultimately the ocean; it is the interaction municipal hydrology and municipal hydraulics engineering directly designs for (sewers, channels, detention).

(ii) IDF Analysis and the Hydrologic Cycle in Major-System Design

Intensity-duration-frequency (IDF) curves statistically summarize the precipitation component of the hydrologic cycle at a location — for each storm duration, the average rainfall intensity associated with a given return period. Because the major system is the overland/emergency route relied on for rare, large storms, its design storm is deliberately drawn from the upper (rare, high-intensity) end of the IDF family — typically the 100-year curve — reflecting the hydrologic-cycle reality that infiltration and interception are quickly overwhelmed at high intensities, so almost all of the rainfall converts to surface runoff. The major system's overland flow paths, road-crown geometry and detention storage are then sized using this near-100%-runoff assumption for the rare event, explicitly because the natural infiltration/evapotranspiration pathways cannot be relied upon to remove much volume during a short, intense, rare storm.

(iii) Hydrologic Cycle Linkages to the Minor System

Two important linkages: (1) Infiltration and antecedent soil moisture control the runoff coefficient used to size the minor (piped) system — a watershed with high infiltration capacity (permeable soils, low imperviousness) generates a smaller fraction of a given rainfall as runoff, directly reducing the pipe sizes needed for the same design storm, whereas urbanization (impervious cover) suppresses infiltration and pushes more volume into the minor system. (2) Evapotranspiration and interception losses reduce the effective rainfall reaching the ground during the early part of a storm (initial abstraction), which the minor system's design storm/runoff-coefficient method (e.g., the Rational Method's $C$) implicitly accounts for; understating these losses oversizes the minor system, while ignoring seasonal changes in vegetation cover (leaf-off vs. leaf-on) can under- or over-estimate the design runoff depending on the season modelled.