PYTR Class

Learning Objectives

1. Analyze how human activities (e.g., agriculture, deforestation, and urbanization) impact the flows and stores of the hydrological cycle.
2. Illustrate the steady-state condition of a water body using flow diagrams of inputs and outputs.

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Part 1: Human Activities

• Human Activities & Hydrological Impact: Agriculture, deforestation, and urbanization can alter water stores and flows. Changes in land use often lead to reduced evapotranspiration and increased runoff, increasing the risk of flash floods.

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Agriculture

• Different types of agriculture impact drainage basin stores and flows differently:
  • Arable (crop) farming: Removes vegetation cover during harvest, making the land more vulnerable to erosion.
  • Pastoral (livestock) farming: Maintains grass cover year-round but may replace diverse forests with monoculture grass, leading to lower interception, reduced storage, and decreased infiltration.
• Vegetation type and density determine interception:
  • Cereals intercept less water than broadleaf crops.
  • Row crops leave bare soil exposed.
  • Forests allow up to five times more infiltration than grasslands by channeling water into the ground.
• Land-use practices impact infiltration:
  • Grazing compacts soil, reducing infiltration.
  • Poor drainage can lead to waterlogging and salinisation.

Table 1: Influence of ground cover on infiltration rates

Ground cover	Infiltration rate (mm h⁻¹)
Old permanent pasture	57
Permanent pasture: moderately grazed	19
Permanent pasture: heavily grazed	13
Strip-cropped	10
Weeds or grain	9
Clean-tilled	7
Bare, crusted ground	6

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Deforestation

• Vegetation helps regulate water movement by increasing interception, reducing overland flow, and boosting evapotranspiration.
• Effects of deforestation:
  • Increased flood risk due to reduced interception and increased runoff.
  • Increased river sediment transport.
  • Increased overland flow leading to more frequent erosion.
  • Reduced infiltration and evapotranspiration.
• Deforestation has widespread impacts as it often occurs for:
  • Agricultural expansion.
  • Industrial and tourist developments.
  • Urbanisation.

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Urbanization

• Urban areas create impermeable surfaces (roads, roofs, pavements) that reduce infiltration and increase runoff.
• Urbanization increases drainage density with extensive networks of drains, gutters, and underground sewers.
• Greater impact in lower drainage basins, where more urban areas are located.
• Conflicting impacts on hydrological processes:
  • Increased erosion due to higher water volumes in rivers.
  • Faster flow and transport of materials due to enlarged channels.
  • Reduced erosion in some areas due to riverbank protection schemes.

Part 2: Steady State of Water Bodies

• Steady State of Water Bodies: The balance of inputs and outputs in a water body can be represented using flow diagrams.
• Uses of Flow Diagrams:
  • Help calculate sustainable rates of water harvesting from lakes, aquifers, and other water bodies.
• Inputs into a Drainage Basin:
  • Mainly from precipitation.
  • Can also come from transfer from other drainage basins and groundwater flow.
• Outputs from a Drainage Basin:
  • Evaporation and transpiration.
  • Discharge out of the basin.
  • Groundwater flow into other drainage basins.
• Examples:
  • Water balance. in the Amazon rainforest.

Exercises