W2 - Hydro - Importance of Precipitation

Importance of Precipitation

• Precipitation contributes to Evapotranspiration, which includes transpiration from plants, interception, throughfall, and evaporation.
• Precipitation also contributes to groundwater recharge and capillary rise from the water table.
• Precipitation is important for runoff, which includes surface runoff, interflow, and baseflow, leading to saturation excess runoff (Dunne) and infiltration excess runoff (Horton).
• It is relevant for water resources planning and management for human use
• Important for food production and ecosystem consumption.
• Precipitation feeds into energy production, recreational use and other industries
• Precipitation impacts the health of humans and ecosystems, especially for water systems.
• It affects dam design and management as well as drainage networks, bridges, and overall infrastructure.
• Precipitation has an impact on structural design, including foundations, groundwater, drainage, and load considerations.

Types of Precipitation

• Snow forms as ice crystals.
• Hail consists of balls of ice.
• Sleet forms as solid grains of ice.
• Drizzle measures less than 1 mm/hr.
• Light rain measures less than 2.5 mm/hr.
• Moderate rain ranges from 2.5 to 7.6 mm/hr.
• Heavy rain exceeds 7.6 mm/hr.

Influencing Factors of Precipitation

• The main source of precipitation is evaporation from ocean surfaces, not evaporation from continents (which averages less than 10%).
• Large-scale cooling helps bring air to saturation.
• Convective systems contribute through unequal radiative heating/cooling on the Earth's surface and atmosphere.
• Convergence occurs through orographic barriers.
• A region's location relative to the general circulation, latitude, and closeness to a moisture source primarily dictate its climate.
• Orographic barriers often play a larger climate role than proximity to a moisture source.
• Climatic and geographic factors combined dictate the amount of atmospheric moisture over a region.
• Precipitation is determined by frequency and type of precipitation-producing storms.

Precipitation Formation

• Evaporation over ocean surfaces is essential
• Formation involves; cooling of air by lifting, saturation of the air, condensation in the presence of nuclei, growth of water droplets by collision and coalescence
• Precipitation begins after critical mass is reached.

Spatial Variability of Precipitation

• Varies greatly across the globe and within smaller regions
• Factors like latitude, elevation, and proximity to large bodies of water influence spatial distribution of rainfall

Temporal Variability of Precipitation

• Hyetographs are rainfall depth vs time
• Hydrographs are rate of flow vs time

Drivers of Spatial and Temporal Variations

• Influenced by cloud patterns, surface/ocean temperature gradients and orography.
• Affected by wind direction, overlying weather patterns, and underlying land surface conditions.

Concept of Infiltration

• Infiltration is the entry of water into the soil surface and its subsequent vertical movement through the soil profile (Brustaert, 2005, p.307).
• Water infiltration continues if the soil has available pores.
• If the soil is saturated, infiltration reaches a minimum, and water begins to run off.
• Infiltration rate is the current rate of water infiltration in the soil, measured in units similar to velocity (length/time).
• Infiltration capacity relates to the defined upper rate limit
• Infiltration capacity is equal to or exceeds infiltration rate.

Darcy's Law

• Helps to provides way to analyses the flow of water in soil
• Equation is q = -k∇h, where q is water flux, k is saturated hydraulic conductivity, and ∇h is hydraulic gradient.
• Applies only to saturated flows.
• Equation cannot be applied to unsaturated flow.
• The infiltration capacity depends on soil type and saturation level.

Unsaturated Flow

• Soils are often not saturated
• Soils can be classified as sand, silt, or clay
• The runoff (R) equals P-I (precipitation - infiltration)
• Infiltration (I) = βP, where β depends on soil moisture and the initial infiltration rate
• Runoff R = (1 – β)P

Water Balance Equation

• The water balance equation is ds/dt = P – R – ET.
• P is observable quantity
• ET = f(SM, atm) and R = f(SM, P, ...), where SM is soil moisture.
• Quantifying R is useful because ET may be small over short periods.

Horton's Infiltration Capacity

• Water supply at the soil surface is equal to or greater than infiltration capacity.
• The infiltration rate can reach a maximum.
• Horton's equation addresses changes in infiltration capacity over time.
• The infiltration capacity changes according to soil condition and water content.

Infiltration Considerations

• If precipitation is less then saturated infiltration infiltration occurs unhindered.
• If precipitation is equal to saturated infiltration : infiltration proceeds unhindered until it reaches minimum infiltration at all times
• If precipitation exceeds saturated infiltration: infiltration occurs in a unhindered way and then saturates the soil

Horton's Infiltration Capacity Equation Parameters

• Simplied using f = a + (b − a)e-ct and f(t) = fc + (fo - fc)e-c
• f is infiltration capacity [LT-1], a, b, c are physical and empirical constants, and t is time [T].
• A parameter within means the saturated infiltration rate [LT-1]
• b is the initial infiltration capacity [LT-1].

Horton's Infiltration Capacity Definitions

• Defines an fo/fc infiltration ratio
• Considers soil type and level of saturation
• The resulting ratio of fc is higher for fine material soils than coarser ones

Caveats to Horton's Model

• Must assume that there is a water source that is greater then the upper boundary condition fo.
• Precipitation must be greater then the initial infiltration rate
• Infiltration is affected by capacity limitations and any runoff must be attributed to excess infiltration
• Precipitation rate is assumed to be smaller then the infiltration capacity (t)

Derivation of Horton's Total Water

• The slope occurs on therecession curve
• Equation: f(t) = fc + (fo - fc)e-ct

Water Yield

• Water management authorities need runoff values to determine water harvesting
• Used for flood prediction and or irrigation
• Apects like vegetation or the soil condition affect overall water yield
• Change in factors must be taken into account
• Water yield is not an aquifers water yield.