3.1: Hydrologic Cycles
Preserve and restore natural hydrologic cycles in the Basin’s watersheds. Other Information:
From absorption into the atmosphere as a gas, to the eventual recharging of lakes, streams, rivers, estuaries and aquifer
systems, the natural hydrologic cycle is a continuous and dynamic process in equilibrium. Human disruption alters the cycle
in many ways. Mitigating these disruptions requires us to address land development practices and regulation, as well as impacts
from existing development. Approximating natural seasonal flow regimes. In addition to gauging water budgets on a reasonable
watershed scale (see the sidebar on page 20 “Defining the Appropriate Scale for Assessing Watersheds”) hydrographs should
be established that reflect a natural range of flow variability. The goal is to develop and manage land in a manner that will
approximate natural seasonal flow patterns. Preserving soil health. Healthy soil infiltrates rainfall and enables recharge
to support stream base flows and ground water supply. Disturbance or removal of soil inhibits infiltration and, if the remaining
soils are compacted, may even prevent recharge from reaching water supply aquifers. Maximizing infiltration reduces runoff
and thus minimizes flooding. Maximizing natural vegetation. Vegetation helps to maintain soil stability, soil structure, and
local temperature regimes (microclimates). When vegetation is removed, soil erosion and sedimentation in waterways increases.
The removal of root systems can exacerbate soil compaction, affecting infiltration and recharge. A loss of vegetation also
alters the amount of evaporated and transpired water to the atmosphere that normally occurs during photosynthesis, resulting
in increased localized temperatures. This “heat island” effect is further intensified by large amounts of paved surfaces.
Not only is the protective shade of streamside vegetation lost, but the stormwater runoff from paved, un-shaded surfaces has
a higher temperature to which native stream species may not be tolerant. Replicating the natural timing and volume of stormwater.
Routing runoff from paved areas directly into waterways interrupts infiltration. This can result in flooding, severe erosion,
temperature and quality changes, and diminution of water availability. Impervious cover and some stormwater collection practices
increase the volume and rate of runoff from storm events and may cause localized changes to ground water. Stormwater management
must counter the flow and recharge alterations caused by development and ensure the recharge volumes equal those of natural
conditions. • Wetlands play an important role in the water cycle and in habitat provision. Landscape preservation and stormwater
management programs need to consider the water requirements to maintain wetland survival.
Objective(s):
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