The Local Water Budget L4

The Local Water Budget and River Systems

Learning Objective

  • To understand how the hydrological cycle influences water budgets and river systems at a local scale.

Key Terms and Definitions

  • Field capacity: The maximum capacity of moisture that a soil can hold.

  • River regime: The annual pattern of flow within a river, influenced by climatic conditions and the characteristics of the drainage basin (both physical factors and human interventions).

  • Effective rainfall: The amount of precipitation remaining after evaporation.

  • Recharge: Occurs when soil moisture levels increase as a result of precipitation following a dry period.

  • Water balance equation: A method for expressing the water budget, balancing precipitation (P), runoff/river discharge (Q), potential evapotranspiration (E), and soil moisture and groundwater storage (S).

    • Full Equation: P = Q + E \pm S

  • Positive feedback: (To learn for Lesson 6)

  • Negative feedback: (To learn for Lesson 6)

  • Resilience: (To learn for Lesson 6)

  • Tipping point: (To learn for Lesson 6)

  • Water deficit: (To learn for Lesson 6)

The Water Budget

  • Definition: The annual balance between hydrological inputs and outputs for a given area.

  • Equation Components:

    • P (Precipitation): Water falling from the atmosphere to the Earth's surface.

    • Q (Channel discharge / Runoff): The volume of water passing a certain point in a river channel over a specific amount of time, typically measured in cumecs (m^3/s).

    • E (Evapotranspiration): The combined effect of evaporation (water turning into vapor from surfaces) and transpiration (water vapor release from plants).

    • S (Change in storage): The fluctuations in the amount of moisture held in the soil (soil moisture content) and underground (groundwater).

  • Significance: This equation allows for understanding the difference between natural water supply and demand. It helps identify periods when precipitation exceeds evapotranspiration (a surplus) and vice versa (a deficit).

  • Key Inputs:

    • Precipitation

    • Water diversion into the area

    • Groundwater flow into the area

    • Surface water flow into the area

    • Surface runoff into the area

  • Key Outputs:

    • Evapotranspiration

    • Water diversion out of the area

    • Groundwater flow out of the area

    • Surface water flow out of the area

    • Surface runoff out of the area

    • Industrial or residential uses within the area

Interpreting the Annual Water Budget Graph

(Labels refer to a typical graph showing mean monthly precipitation and evapotranspiration)

  • A (Precipitation Exceeds PE, Soil Moisture Full): Precipitation (P) is greater than potential evapotranspiration (PE). Soil water stores are at their maximum (field capacity). There is a soil moisture surplus, contributing to runoff and groundwater recharge.

  • B (PE Exceeds Precipitation, Soil Moisture Utilisation): Potential evapotranspiration (PE) is greater than precipitation. Existing soil moisture is used up by plants or lost to the atmosphere through evaporation. This process is known as soil moisture utilisation.

  • C (Soil Moisture Depleted, River Levels Fall): The soil moisture store is now depleted. Any new precipitation is likely to be absorbed by the dry soil rather than generating runoff. River levels will typically fall or may even dry up completely.

  • D (Deficiency of Soil Moisture): A significant deficiency of soil moisture exists as the store has been fully utilised. PE continues to exceed precipitation. Plants must adapt to survive these dry conditions, and crops often require irrigation.

  • E (Soil Moisture Recharged): Precipitation begins to exceed PE once again. The soil moisture store starts to be replenished.

  • F (Field Capacity Reached, Groundwater Recharge): Soil moisture is now full, having reached field capacity. Any additional rainfall will percolate downwards to the water table, leading to the recharge of groundwater stores.

Seasonal Variations in Water Budget (UK Context)

  • In the Summer (e.g., UK):

    • Potential evapotranspiration (PE) typically exceeds precipitation.

    • This leads to lower water levels in rivers, lakes, and ponds.

    • Soil moisture content declines, causing vegetation to wilt.

    • By August, a notable water deficit often occurs.

  • In the Winter (e.g., UK):

    • The UK’s ‘water year’ conventionally begins in October, when rainfall usually starts to exceed evaporation.

    • Storage areas (soil moisture and groundwater) are generally recharged by January.

    • After January, a water surplus often develops, which increases the risk of flooding.

River Regimes

  • Definition: The annual pattern of a river’s flow, which can be graphically represented by a storm hydrograph at a local level.

  • Simple Regimes:

    • Characterised by distinct periods of seasonally high discharge followed by low discharge.

    • Often found in mountainous regions where summer snowmelt is a significant factor.

    • Example: The Rhône and the Arve rivers in the Alps exhibit simple regimes.

  • Complex Regimes:

    • Occur when a river traverses multiple distinct relief and climatic zones.

    • Human factors also play a significant role in their complexity.

    • Generally, the longer the river, the more intricate its regime tends to be.

    • Examples: The Yukon (Alaska), the Amazon (Brazil), and the Murray-Darling (Australia).

Named Examples of Complex River Regimes

  • Yukon River, Alaska, USA:

    • Flow Patterns: High flow occurs during spring and summer, primarily due to snowmelt. Low or no flow is experienced in winter when precipitation is frozen. Exhibits very large seasonal variability.

    • Human Influences: Relatively few, as most of its landscape is wilderness. Some hydroelectric power (HEP) use supports mining industries.

    • Major Influences: Dominated by tundra, taiga, and mountain climates. Higher summer temperatures, rainfall, and snowmelt coincide to create peak flow.

    • Peak flow: June.

    • Low flow: November.

  • Amazon River, Brazil:

    • Flow Patterns: High flow occurs during the wetter season, and low flow during the drier season. Displays moderate seasonal variability, often fed by Andean rivers outside the main rainforest region.

    • Human Influences: Increasing, although human abstraction still accounts for a low percentage of its overall flow. Large dams are used by Brazil's major cities for irrigation and HEP.

    • Major Influences: Predominantly a rainforest climate. Experiences seasonal precipitation, with rainfall every month but distinct higher and lower rainfall periods. Evapotranspiration levels are very high.

    • Peak flow: May-July.

    • Low flow: November.

  • Murray-Darling River, Australia:

    • Basin Characteristics: Encompasses over 1 \,000 \,000 \,km^2 with diverse geography, from rugged mountains to flat semi-arid plains. Climates range from sub-tropical in the north, semi-arid in the west, to temperate in the south.

    • Rainfall Patterns: Rainfall transitions from summer-dominant in the north to winter-dominant in the south. The eastern side receives high average annual rainfall (up to 1 \,500 \,mm), including winter snow in the Great Dividing Range peaks. The western side is typically hot and dry, with average annual rainfall generally less than 300 \,mm.

    • Evaporation Rates: Extremely high, with 94\% of rainfall evaporating from the land and surface water or being transpired by plants. Contributing factors include low-lying topography, warm to hot semi-arid conditions across most regions, and the meandering, slow-flowing nature of its creeks and rivers.

    • Flow Patterns: High flow during the wet season and low/no flow in the dry season. Exhibits high seasonal variability.

    • Human Influences: Its waters are extensively drawn by Australia's major cities and farms for irrigation. Years of water siphoning for irrigation have led to the devastation of waterways.

    • Major Influences: Seasonal sub-tropical climate, with a monsoon climate in northern Queensland tributaries (feeding the Darling) and a temperate climate in the south (feeding the Murray). Most of the basin lies in a rain shadow and experiences long periods of drought.

    • Peak flow: August-September.

    • Low flow: February-April.

European River Regimes

  • Figure 5.10 illustrates various river regimes across Europe, showing specific discharge in l/second per km^2, demonstrating diverse annual flow patterns influenced by local climatic and geographical conditions.

Sustainable Drainage Systems (SuDS)

  • Purpose: SuDS are methods and strategies introduced to reduce the volume and rate of surface runoff generated from rainfall, particularly in urban and developed areas.

  • Examples of SuDS:

    • Rain gardens: Designed to store and filter water in urban landscapes.

    • Green roofs: Vegetated roof systems that absorb and store rainwater, reducing runoff (e.g., on holiday homes at Widemouth Bay in Cornwall).

    • Permeable paving: Allows water to infiltrate through its surface into the ground beneath, reducing surface runoff.

    • Detention basin: Dry basins designed to temporarily hold excess runoff during storm events and release it slowly.

    • Wetlands: Constructed or natural wetland areas that store, filter, and treat stormwater runoff.

Planners and Flood Risk (UK Context)

  • Within the UK, government planners are legally required to assess whether any proposed development (e.g., a new housing estate or industrial facility) will impact or increase the flood risk in the area.

Universal Use of SuDS: Discussion Points for Planners

  • Benefits of SuDS: Their effectiveness in preventing or mitigating flooding.

  • Cost: The expense and financial implications of implementing SuDS on a wide scale.

  • Monitoring: The feasibility and challenges in monitoring the effectiveness and maintenance of SuDS.

  • Appropriateness: Whether SuDS are suitable for all types of built environments and geographical contexts.

  • Resilience to Extreme Events: The capability of SuDS to manage and mitigate the risks posed by extreme events, such as 100-year floods.

Summary of Key Learnings

  • The water budget quantifies the balance between hydrological inputs and outputs in a specific geographical area.

  • Water availability is significantly influenced by factors such as soil moisture levels and prevailing land use practices.

  • The capacity of soil to retain moisture (field capacity) is a critical component of the local hydrological cycle.

  • A river's regime describes its annual flow pattern, which is a reflection of local differences in precipitation, temperature, evapotranspiration rates, and land use.

  • A storm hydrograph illustrates a river's response to a particular storm event, with its shape being determined by both physical characteristics of the drainage basin and human interventions.

  • UK government planners have a responsibility to evaluate and determine the potential influence of any proposed development on the area's flood risk.