Chapter 1 · Unit 1-1
A raindrop's journey from the clouds back to the sea
Textbook p. 3–6
Goals
Key terms
1.1 Definition of hydrology
Hydrology is the earth science of the occurrence, circulation and distribution of water on Earth, its physical and chemical properties, and its relation to all living things.
Engineering hydrology
An applied earth science: it uses hydrologic principles to solve the engineering problems of water-resources development. It models the total volume and timing of water in the hydrologic cycle to set design criteria, and it assesses the risk each structure faces.
1.2 Hydrologic cycle
Earth's water circulates continuously through the atmosphere, soil and oceans.
The simplest version: seawater absorbs solar energy and evaporates → the vapor condenses into precipitation → rain, snow or frost falls on land → gravity carries it through streams and rivers back to the sea.
Mountains on the left, sea on the right; the front is cut open to show the subsurface. Drag to rotate.
Liquid transport · 1 / 11
Water vapor condenses into clouds, then falls from the atmosphere to the ground as rain, snow or frost.
Coming up: why do clouds often gather over mountains? Chapter 3 explains the causes of rainfall.
Temporary storage · 2 / 11
Falling rain is first caught by tree canopies or buildings. This is interception.
Part of the intercepted water evaporates straight back to the air (Fig. 1-2: interception → evaporation E) and never reaches the ground; the rest drips from the canopy.
Temporary storage · 3 / 11
Water flowing over the land collects in hollows in the ground. This is depression storage.
This water later evaporates or slowly infiltrates into the soil (Fig. 1-2: depression storage → infiltration, plus evapotranspiration ET).
Liquid transport · 4 / 11
Part of the rain that reaches the ground soaks into the soil from the surface. This is infiltration.
Look at the front cut: light-blue particles move down from the surface through the dark-brown topsoil. The bright blue line on the cut is the water table.
Liquid transport · 5 / 11
Rain that does not infiltrate flows over the land surface as overland flow.
Once it passes through the channel network into rivers, it is called surface runoff. The particles gather downslope into streams and rivers, and finally reach the sea.
Why does rain sometimes produce no overland flow? Try it in the interactive lab on slide 16.
Liquid transport · 6 / 11
Some infiltrated water flows sideways through shallow soil before it reaches the water table, then leaves the ground into a river. This is interflow.
The stream on the cut: interflow from the left hillside seeps into it through the bank.
To groundwater · 7–8 / 11
Some water percolates straight down through the unsaturated zone to below the water table.
Water that moves through the aquifer and finally enters a river (or the sea) is groundwater. It moves slowly, so rivers keep flowing in long droughts. Chapter 2 calls it baseflow.
Liquid transport · 9 / 11
When the topsoil is dry, soil water moves up from deeper layers to the surface. This is exfiltration.
The opposite of infiltration: infiltration goes surface → soil; exfiltration goes soil → surface.
Vapor transport · 10–11 / 11
Evaporation: in strong sunshine, water molecules in soil or open water (depressions, rivers, lakes, oceans) absorb solar energy and turn from liquid to vapor.
Transpiration: soil water rises through plant roots to stems and leaves, then escapes to the atmosphere. Fig. 1-2 combines the two as evapotranspiration ET.
Wrap-up
Press → to sort the 11 terms into the textbook's two lists.
Vapor transport
Liquid transport
In neither list
Interception and depression storage are temporary storage; percolation is the path water takes to groundwater.
Fig. 1-2 redrawn
The input at the top is rainfall intensity over time; the output at the bottom is the outlet hydrograph. Each box in between is a process.
Interactive lab
The sliders on the right control the rain and the soil; the 3D view, the equation and the hydrograph below change together. Try to:
04 Rainfall & hydrograph · Figs. 1-2, 2-9
Surface runoffInterflowGroundwaterInfiltration capacity f
Self-check
01Rain soaks into the soil, then flows sideways into a river before reaching the water table. What is this called?
02Which of these is vapor transport?
03During a storm, P = 120 mm, infiltration INF = 58 mm and surface runoff Q = 57 mm; ignore E and T.
By Eq. (1-2), what is the change in surface storage ΔSs?
Summary
Next unit
1-2 The Water Budget Equation
Write this unit's water budget as Eqs. (1-1)–(1-4) and solve Example 1.1.