BNPP/ASB Functional Value of Biodiversity Project – Phase II
Appendix
5
Download Appendix 5
Terminology
used for Watershed functions
The
broad category of ‘watershed functions’ may well be the first
‘environmental service functions’ that has been recognized as such,
and it continues to be the one with the largest immediate relevance for
people, especially for poor people who don’t have the opportunities of
the better-off to shield themselves from the impact of droughts, floods
and poor quality of water. With strongly increasing demand for water and a
constant supply, the prediction that conflicts over water are likely to
increase is easily justified.
A simple way to explore the overall concept
of ‘watershed functions’ is first of all to look at the hydrological
‘outcomes’, in this case the flow of water coming out of an area in
rivers, and sometimes in subsurface groundwater flows. We can distinguish
(see ASB lecture note 7) between the
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Quantity or total water yield
-
Evenness of flow, which implies high flows in the ‘dry’ season
and an absence of strong peak flows in the set season
-
Quality of water, with respect to its use as drinking water, other
domestic uses, industrial use, irrigation or as habitat for fish and other
water organisms
These
three aspects are influenced by land use to different degrees, and this
has consequence for possible ‘reward’ mechanisms.
Rainfall
varies between different parts of the earth, from approximately 0 to over
10 m of rainfall per year (that means that if rainfall would not
infiltrate the soil or runoff laterally a lake of 10 m depth could be
formed in a year, in the absence of evaporation at the surface of the
lake). Rainfall is usually expressed in mm rather than m, and is broadly
linked to the type of natural vegetation: evergreen tropical forest
usually requires rainfall amounts of more than 1500 mm year-1,
deciduous (= shedding leaves in an ‘off’ season) forest and savanna
may grow in the 800 – 1500 mm year-1 range, and
various forms of scrub or open vegetation in the 300 – 800 mm year-1
range. Below 300 mm year-1 very
few
crops can be grown without irrigation, and the natural vegetation will
consist of short grass or desert specialists. As forests are associated
with high rainfall, it may come as no surprise that the cause-effect
relation has been confused: do forests cause rainfall? Or does rainfall
allow forests to grow?. The perspective that deforestation will lead to a
reduction of rainfall has a long history (elegantly reviewed by Williams
in his book ‘Deforesting the Earth’), but remarkably little hard
evidence in its support, despite the large scale at which the
‘experiment’ of deforestation has been implemented, first in Europe,
than in North America and currently in the tropics.
Current evidence points to clear relations at global scale, with atmospheric circulation and thus rainfall zones shifting even if the total may stay the same. Some places definitely have become wetter, others drier, and future changes may add to variability, even if the direction of change for specific locations is not clear yet. These real changes in climate have coincided in many parts of the tropics with real changes in forest cover – even though the causal link is indirect, via global climate change. The continued perception of a direct link is thus understandable, but a real effects is unlikely to be large, if it exists at all. If we take for granted that effect of local land use on total annual rainfall are small, the main effect on total water yield of a catchment area is a change in the rate of evapotranspiration, or the return flow of water molecules to the atmosphere. In a simple equation: Q = P – E, or the total water yield (surface rivers+ groundwater flows) equals precipitation (rainfall plus snow and ice, which in most parts of the tropics can be ignored) minus evapotranspiration. That leads to the scheme in figure App5.1.
Figure App5.1. Schematic relations of ‘water quantity’ as landscape outcome
Four
classes of land cover can be distinguished from the perspective of evapotranspiration:
-
open water bodies, where water loss is determined by the relative humidity of the air and the presence of a stagnant boundary layer of air that reduces the transport of water vapour,
-
open soil, which may have a rate of evaporation similar to open water bodies when the surface is wet, but where evaporation may rapidly become limited by the rate of transport to the soil surface; soil cover with a litter layer provides a stagnant air zone, further reducing transport opportunities and mixing with the atmosphere
-
seasonally green vegetation: most plants are able to provide their leaves (evaporating surfaces) with the amount of water that is needed for evaporation similar to an open water surface, during most of the rainy season; during periodic dry spells, plant transpiration is likely to drop below the value of open water, but stay above that of open soil,
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evergeen vegetation such as evergreen trees (e.g. pines, eucalypts, trees such as grevillea), irrigated rice paddies or vegetable crops will have a rate of transpiration equal to that of open water, or higher if lateral flows of dry air drive the evapotranspiration per unit area to higher levels.
Efforts
of land users that will reduce evapotranspiration and thus increase total
water yield may thus be found in not planting evergeen trees
(especially fast growing ones), or irrigating rice paddies or vegetable
crops in the dry season.
The
differences in total water use between different types of vegetation
(deciduous or evergreen) are often less than 300 mm year-1. In
a climate zone with a n annual total of 1500 mm year-1, such a
difference is likely to be noticeable (and many villagers complain that
reforestation with pine trees or eucalypts reduces dry season flow or
total water yield – even though the public and forest service tends to
believe that such trees will increase water yield). In climates with
higher rainfall the same absolute difference will be smaller relatively
speaking., and may drop below the threshold of what people can notice and
care for.
Overall
we can say that the total water yield of any ‘catchment’ area is
largely determined by rainfall and thus outside of the control of any
local land users. The difference that land cover can make is fairly well
bounded (less than 300 mm year-1), and rewards for efforts may
have to focus on this difference against baseline, rather than at the
total volume that actually comes out of a watershed (unless one attributes
a greater influence to ‘human rainmakers’ than most of them would
subscribe to themselves).
Total
water yield per unit rainfall can be used as an indicator, with a
value that increases more than proportionally with total rainfall. The
potential effect of land use change is likely to decrease
more than proportionally with total rainfall.
Evenness
of water flow and buffering
Floods
alternating with droughts – that is the general picture of ‘disturbed
watershed’. When we make a comparison across the tropics, however, we
see that not only the total amount of rainfall per year varies over more
than two orders of magnitude (i.c. from 0.1 – 10 m year-1),
but also the variability: the number of dry and wet months can vary quite
independently of total rainfall (giving rise to various climate
classification schemes that use the number of dry and wet months rather
than total rainfall). Evenness of riverflow, in the sense of a
continuation of flow during dry months and an absence of high peaks and
floods in wet months, may thus be largely attributed to the local climate
– and thus to the ‘natural capital’. Land cover, and thus the
decisions of local ‘actors’ will influence the degree of
‘buffering’, but we need to carefully tease out the part that can be
influenced, if we want to get a clear basis for ‘rewards’.
A straightforward way to define ‘buffering’ is to compare the total quantity of river flow at above-average rates, with the total quantity of rainfall at above-average rates. Buffering equals 1 – the ratio of these two quantities, both expressed in mm year-1. As daily rainfall data are most widely available, we can take this timestep as a basis for the calculations of what is above average riverflow or rainfall. A fully ‘asphalted’ watershed where riverflow directly follows rainfall may have a buffering of 0, a watershed that provides constant riverflow regardless of the rainfall pattern has a buffering of 1. A real watershed will be in between these two extremes.
Figure App5.2. As figure App5.1, but specific to the ‘evenness of water flow’ function
With
this definition of buffering, we can further analyze a range of
influences. Land cover is important, especially where it influences the
rate of infiltration of rain into the soil, by maintaining a good soil
structure (one can argue whether it is the earthworms that do this, the
trees that feed the earthworms, or the farmers that plant the trees, but
that is another story). But the basic make-up of the landscape, the depth
of soil over bed-rock, the slopes, and the type of soil (soil texture,
specific soil horizons that don’t allow water to penetrate, all
influence the degree of ‘buffering’. A further influence on
‘buffering’ is the degree of spatial correlation of rainfall: where
rainfall is dominated by ‘fronts’ large areas may receive rainfall on
the same day; where (convective) thunderstorms dominate, a strong
‘patchiness’ of rainfall may cause different streams to carry water at
different days and a river that integrates across these streams to be
relatively stable – even without forest cover. Buffering, according to
our definition, will thus depend on the location of the observer relative
to the watershed. The further away, the more even the river will tend to
be, and the less obvious effects of land use change may be. Current
research is trying to quantify these relations, but empirically good
evidence for changes of land use on evenness of flow exists for catchments
up to 100 km2 and little or none for catchments of more than
1000 km2.
With
current hydrological models it is possible to determine which part of the
overall degree of ‘buffering’ that an observer at a certain distance
from a ‘catchment area’ will perceive can be directly related to the
land use in the catchment, with a specific role for the riparian
vegetation in and around the riverbed. Slow transmission of water, linked
to trees and dead wood in the channel, may cause local flooding, but
increases the evenness of flow of a downstream observer (again clarifying
that we need to be explicit about the point of observation or the location
of the stakeholders before we can quantify ‘evenness of flow’).
An efficient way of presenting the input and output of a watershed area in a single graph, is to look at the exceedance probabilities for daily rainfall, daily evapotranspiration and daily riverflow. If a sufficiently long time period is considered (at least 1 year), changes in storage in soil, groundwater and surface water may be negligible and the areas to the left of the curves for rainfall and evapotranspiration + riverflow should be approximately equal. The point of intersection has to have an X-value that equals the mean daily rainfall. The intersection would be at an exceedance probability of 0.5 if rainfall distribution were symmetrical and there would be no dry days – in reality skewness of rainfall distribution plus the fraction of days without rain cause the point of intersection to have a value on the Y-axis that is above 0.5.
In an ‘asphalted’ watershed, the riverflow curve may be expected to coincide with the rainfall curve and there is no buffering. In an ideally buffered situation the riverflow may be constant and equal to the mean at every day of the year. In between these two extremes we’ll find real watersheds with a partial ‘buffering’.
A quantitative indicator of ‘buffering’ (0,1) can be derived as
If, hypothetically, rainfall would be constant, the watershed will not be able to express any ‘buffering’, and the buffering would be zero. With this definition we can explore ‘buffering’ as the resultant of:
1.1.1.1 Site
- local rainfall regime (and its temporal autocorrelation)
- underlying landscape and geology that determines release of groundwater
1.1.1.2 Scale
- size of the catchment (upstream of the observer/stakeholder) relative to the spatial autocorrelation of rainfall
1.1.1.3 Land use
- infiltration and supply to groundwater as potentially influenced by vegetation and land use
- the properties of the riverbed (and temporary storage) that dominate pulse transmission
1.1.1.4 Engineering
- any regulating structures or dams in the river
We can thus separate
the ‘buffering component’ that is attributable to land use (and thus
to human ‘environmental service providers’) from those
that ‘come with the territory’ but do not reflect any
specific effort (and thus form no basis for ES function rewards).
Water
from forests streams can be directly suitable for drinking, if one can be
sure no people live upstream. Otherwise, surface water is hardly ever
directly suitable for drinking – even if many people in rural areas are
in fact relying on it. Water from wells that tap into subsurface flows of
water or groundwater may be safe, as long as the filter effect of the soil
surrounding the well is not overcharged. Below the standards for safe
drinking water, a range of other uses have less stringent criteria for
quality:
-
other domestic use
-
fishponds and drinking water for domestic animals
-
industrial processes
-
irrigation
-
cooling systems
-
filling a reservoir for future use (but allowing sedimentation and
other changes in water quality to occur)
Where water from watersheds with natural vegetation may meet the criteria for all, human activity in watersheds may decrease water quality before it has any substantial effect on the other watershed functions. Where point sources of water pollution can be many orders of magnitude above the detection capacity, it is understandable that long range effects of land use on water quantity have been recorded, at least to catchments of 105 km2. Pollution of water can be a consequence of mining (especially where mercury (Hg) or cyanide (HCN) are used for gold mining in riverbeds…), use of pesticides and fertilizer (especially in the quantities often used on vegetable crops) and people living around streams and using the streams for personal hygiene. More directly linked to land use, erosion in its various forms (sheet erosion, gully erosion and collapse of river banks) can increase the ‘sediment load’ of rivers. Disturbance of groundwater flows by agricultural crops that use less water than the native vegetation that they replaced can bring salt into circulation, especially in drier climates with deep salt deposits.
‘Absence
of threats’ is thus the key way to provide the ‘watershed function’
of delivering clean water. For some forms of pollution, especially where
‘sediment loads’ are due to sheet erosion, vegetation around streams
and rivers, in the riparian zone, can perform a (partial) filter function
and reduce the load of the river. Increasing the effectiveness of such
filter vegetation can thus, under specific circumstances, be seen as
‘enhancing watershed functions’.
A wide range of measurable indicators of water quality is available and mostly used for testing the safety of drinking water. River water of very low quantity can still be made suitable for consumption by technical means, relying on filtration in sandbeds, aeration and specific chemical processes. This ‘end of pipe’ solution can be used as a point of reference for the economic valuation of the provision of clean water (that requires less intensive or no treatment).
The
general public and policy perception of ‘watershed protection’ does
not rely on the previous three outcomes, but rather specifies a desirable
condition within the watershed (usually ‘forest’) – with all
reductions in forest cover associated with a loss of ‘watershed
functions’.
The
clearest functional relation between trees (especially deep rooted ones)
and the integrity of watersheds is found in the prevention of landslides.
Landslides can occur on any slope if the weight of a soil column after
heavy rainfall is greater than the ‘sheer strength’ or the resistance
to movement. Deep rooted trees can provide ‘anchoring’ of soil layers
and prevent their movement. When the trees are cut (especially in a
‘clear cut’ affecting all trees on a slope)
the propensity for landslide will increase – especially when
after a few years, the deep roots decompose. Many landslides, however, are
linked to road construction cutting into slopes and interfering with the
mechanical stability. Landslides are common in natural vegetation on steep
slopes (and geologically young or volcanically active mountain areas), but
are usually interrupted by vegetation downhill that can act as a
‘filter’. During earthquakes or extreme rainfall for several days,
such filters may loose their effectiveness. After forest clearing,
landslides can more easily
increase in size, and lead to major mudflows destroying everything in
their path. Reducing human losses from landslides can be achieved first of
all by not building houses in vulnerable sites. In general, avoiding clear
felling of forests on slopes will reduce landslide risk. A substantial
length of time of observation may be needed, however, to actually prove
changes in ‘landslide risk’.
‘Erosion
control’ is often included in lists of
watershed functions, and as a positive attribute of forests. In
evaluating this as an ‘environmental service function’, we need to be
careful. Erosion tends to reduce the future fertility of the eroding site
– but this will be the immediate concern of the farmer on the site,
rather than outside stakeholders. Similar to the ‘existence value’ in
the biodiversity function, one can argue that knowledge of the
preservation of topsoil has value to outside stakeholders. Further
rationalizations of such value can be derived from the need for farmers to
clear further forest lands as a consequence of loss of on-site
productivity. The causal chain in these cases is rather complex. In the
absence of filter vegetation surrounding the plot, or in the pathway
between plot and stream, erosion can increase sediment load of the river
and thus reduce water quality.
While
erosion rates under most types of forests are low, there are some notable
exceptions in forests that do not have an understory or permanent litter
layer. Drips falling from a tree canopy after rainfall can actually have a
higher splash impact on the soil and lead to greater erosion than would
have occurred without (plantation) forest. A simple criterion for absence
of erosion is the presence of a litter layer. This works in two ways: it
is an indicator that there is little overland flow (otherwise the litter
would be washed away) and it contributes to the activity of soil biota
that maintain soil structure and infiltration rates for water. The
watershed function ‘prevention of erosion’ may thus be better linked
to the litterlayer than to the presence of trees as such.
Overall, we can conclude that the holistic concept of ‘watershed functions’ that require ‘intact forest’ and ‘absence of human activity’ refers to only one way of maintaining measurable outcomes in the range that is acceptable to downstream stakeholders. Depending on the rainfall, landscape properties and the distance to the watershed area, quantity, evenness and quality of the water in the river can be maintained in landscapes that are used for agricultural production. Key locations for maintaining forest cover are: tops of the ridges and hills if clean groundwater is important and riparian forests for filter functions and slowing flood pulse propagation. Outside of these two ‘keystone’ locations, we may need enough tree cover to maintain a permanent litter layer and thus infiltration conditions, but the need for this depends on soil type (propensity to loose its structure and infiltration capacity) and rainfall distribution.
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