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The theoretical maximum amount of ET that would occur from an area of continuous, uniform vegetation that covers the whole ground - with an unlimited supply of water


Aridity Index

A = P/PET = numerical indicator of the degree of dryness of the climate at a given location
<0.05 = hyperarid
0.05-0.20 = arid
0.20-0.50 = semiarid
0.50-0.65 = subhumid
>0.65 = humid


Evaporative Index

Determines the portioning of P ---> ET and Q and is used in the Budyko Curve


Dryness Index

Characterises climatic conditions and is used in the Budyko Curve


Hydrologic Design

The process of assessing the impact of hydrologic events on a water resource system and choosing values for the key variables of the system so it will perform adequately.


What are hydraulic design estimates based on?

Hydraulic design estimates are based on either the largest event we can imagine (ELV) or the expected return period of an event.


Probability of an event

Inverse of the return period. P(X>Xt) = 1/T


What's the problem with trying to do statistical frequency analysis in developing countries?

It requires a lot of data and developing countries are generally data poor


Return Period

Average length of time between occurrences of a storm of a given magnitude or greater. Also known as recurrence interval. An estimate of the liklihood of an event.


What is a streamflow gauge?

A streamflow gauge is a device/structure that continuously measures the discharge in a river


Describe the processes of the Hydrological Cycle

Precipitation --> Infiltration/Interception/Percolation --> Surface runoff/GW base subsurface flow --> ET and Evap --> Surface outflow/GW outflow --> Evap from oceans.


Residence time

Residence time is the average travel time, Tr, for water to pass through a subsystem of the hydrological cycle. Tr = S/Q


Green water

Water from precipitation that is stored in the root zone of the soil/on top of the soil or in vegetation which is evaporated/transpired/incorporated by plants. Particularly relevant in agricultural and forestry production.


Blue Water

Freshwater, surface water or ground water. It is either evaporated, transferred from one body of water to another or used for the production of a product. Relevant for irrigated agriculture, industry and domestic water use.


Grey Water

Grey water is polluted. It comes about a result of domestic activities


What are the assumptions we use in the water balance equation?

1. Groundwater in is equal to groundwater out. We assume this because we assume that the catchment is very large. And also that hydrogeological characteristics are not that important.

2. We assume that there is no change in storage over long periods of time.


Sources of Evapotranspiration

- Open Water
- Soil
- Vegetation surfaces

- Plants



Collective term, covers all the ways in which (liquid) water is transferred as water vapour to the atmosphere. Includes evaporation from open water (lakes & reservoirs), soil surfaces and water intercepted by vegetative surfaces.



Evaporation of water vapour through the leaves of plants via leaf stomata (pores)


Three types of classification used for the Budyko curve?

Land Cover
Soil Type


What does the Budyko framework do?

Describes the long term water and energy balance of a catchment. It depicts the partitioning of P.


3 reasons for falling off the Budyko curve?

Budyko is more reliable using long-term averages (>>1yr) and large catchments (>10,000km2)

1. Inadequate observations of precipitation, temperature and discharge.

2. Inadequate representation of (potential) ET

3. Inadequate representation of discharge


What are inadequate observations caused by? (reason for falling off Budyko curve)

1. Instrument errors - systematic bias, equipment malfunction, replaced sensors, sensor 'drift' (gets worse over time

2. Lack of spatial coverage - low vs. high elevation, minimum number of sensors per catchment, generally more in lower regions


What causes an inadequate representation of (potential) ET? (reason for falling of Budyko)

ET underestimated shifts right

1. Failure to consider net radiation (insolation)

2. Relative humidity and wind speed for PET and ET computation results in shifts away from the curve.


What causes an inadequate representation of discharge? (reason for falling off Budyko)

GW loss underestimated shifts down

Failure to consider additional groundwater loses of precipitation



The degree to which Q (runoff) is synchronised with P (precipitation).


Responsivity corresponds to a vertical or horizontal gap on the Budyko?

And the bigger the gap the ________ the responsivity?

Vertical, lower


Elasticity corresponds to ______ and ______ changes on the Budyko

A big vertical gap and a small horizontal gap correlates to __________ elasticity

Horizontal and vertical




How quick a catchment can return to normal functioning after perturbations.


What are ground based measurements? And what are 4 examples?

They can support stochastic (random) hydrology and help design and validate hydrological models. Stream flow gauge, radar, distrometer, rain gauge.