Hydrology 2000. IAHS Pubi. no. 171, 1987.
Surface water hydrology
ANDRAS SZÔLLÔSI-NAGY,
ZBIGNIEW W. KUNDZEWICZ &
JOSE CORDOVA-RODRIGUEZ
WHERE IX) 1 1 GO TO?
The idea of creating a list of challenging research areas in
hydrology is not new. It is interesting to look back at earlier
hydrofuturological forecasts and to see how many came true.
Consider the Report of the Committee on Status and Needs in
Hydrology of the American Geophysical Union dated 1.964, where 63
research areas in hydrology were identified. Most of them still
provide challenging research directions. Among the areas mentioned,
those relevant to surface waters that are still in the forefront of
hydrology are: évapotranspiration, effects of land use, channel
behaviour, basin characteristics, hydroclimatic inferences from
natural phenomena, rainfall-runoff relations and areal distribution
of storm precipitation.
Going through this list one could easily say: "we went through
all these items a number of times even before 1964, and we know a
lot about these problems", and one would be right! Another view,
perhaps a bit more theory minded, could equally easily be: "we have
learned very little during the past 23 years as we still have the
very same issues on our shopping list! ", and this view is also
right.
This apparent contradiction is resolved by the concept of a
spiral growth of knowledge. It seems that there will be no
completely new important problems in surface water hydrology before
the end of the century. That is, the major problems known today
will be revisited with modernized theoretical background and
measurement technology.
What directions shall we be taking? Probably we shall be
approaching the unfathomable mysteries of nature by circling around,
knowing more and more and realizing how little we really know.
PROCESSES BEYISITED
We have inherited deficient priorities concerning particular
processes contributing to the hydrological cycle. There are well
over five thousand publications relevant to flow and flood routing,
in comparison with many fewer for several other processes, where the
gaps in our knowledge are greater and where empirical approaches
prevail. Hydrologists like flood routing, since the problem is well
formulated, the processes are well understood and the
signal-to-noise ratio is high compared to hydrological norms.
Therefore, the area of flood routing which lends itself to
9
10
Andras Szôllôsi-Nagy et al.
dissertation writing has been a testing ground for new approaches,
some of which may hold promise for applications to other
hydrological processes.
However a better insight into the hydrological cycle requires
that more attention be paid to other processes, notably
évapotranspiration and precipitation. Small errors in assessment of
the variables in these processes are likely to be amplified when
aggregated in catchment modelling. The statement that "it is more
important what to route, than how to route", although formulated
decades ago, is still not ubiquitously accepted.
Evapotranspiration is one of the least understood processes
contributing to the hydrological cycle. The need is felt to improve
the method of parameterization at larger scales, such as the
watershed or the region. A possible key to progress is better
understanding of turbulent transport mechanisms in the atmospheric
boundary layer (ABL).
Recent years have brought a significant increase of interest in
precipitation processes. This promising tendency should continue.
Studies of precipitation should perceive the process at different
scales, account for interactions among scales and allow aggregation
and disaggregation processes in time and space to be considered.
This is likely to be approached in the following ways:
- Modelling point precipitation process in time. In addition to
one-dimvensional models for one monitoring station there is a
tendency to use multivariate point precipitation models which
preserve the statistical characteristics of the precipitation
process at each monitoring station and also the cross-covariance
between monitoring stations.
- Modelling the processes involved in the origin, development and
decay of a storm event by modelling a storm event in space and time,
involving non-stationary point processes.
- Representing the processes in space and time as a
multidimensional model by attempts to generalize the homogeneous and
stationary point process in order to consider nonhomogeneity and
nonstationarity.
- In addition to all these models, there is an urgent need to
couple the physical laws of meteorology with the precipitation
process in order to produce models with more than just a conceptual
physical basis. This is likely to be the way by which short-term
forecasting methods can be improved.
It seems worthwhile to look for new methodologies that may
improve the understanding of process behaviour (e.g. the concept of
deterministic chaos). It seems to be well established that
turbulence and vorticities involved in atmospheric circulation prove
long-term rainfall forecasts obtained via integrating dynamic
equations of atmospheric circulation to be useless. This relates to
the general theoretical problem of the predictability of flow which
possesses many scales of motion. Each scale of motion has an
intrinsic finite range of predictability. Development of the
situation is affected by the initial and boundary conditions to a
degree which rapidly decreases with time. Prediction error in the
future cannot therefore be reduced by improving measurement values
of the present state-variables.
Surface water hydrology
11
EAIHFALL-EOTOIF EELATIOISHIPS
I t i s l i k e l y t h a t the main field of research in the coming 15 years
i n surface hydrology will continue to be the dynamics of
r a i n f a l 1- runo ff rela tio ns.
Rainfall-runoff modelling, p a r t i c u l a r l y that used for operational
purposes, s t i l l f a i l s to tackle the key i s s u e , which i s an
understanding of how the unsaturated zone changes. Although some
procedures do t r y to r e l a t e the change of soil moisture content to
antecedent p r e c i p i t a t i o n conditions, these procedures are s t i l l
largely a r b i t a r y and crude ( e . g . different threshold models or
continuous API accounting techniques). A major breakthrough i s
required to model surface runoff/soil moisture interactions
properly.
The h o l i s t i c concept c a l l s for a consideration of other forms of
energy such as those associated with thermodynamic and biochemical
processes, in addition to k i n e t i c and potential energy, which are
only taken into account in c l a s s i c a l approaches.
The forthcoming decades w i l l probably witness the end of the era
of single input/single output (SISO) models, whether in the IUH
approach or in the more complicated, yet no more physically based,
Volterra series modelling. The downfall of the SISO approaches w i l l
be brought about by an increase in understanding combined with
improvements in data a v a i l a b i l i t y and in computational power.
Rainfall-runoff models w i l l be increasingly based on physical
•characteristics such as climate, topography, s o i l , vegetation and
degree of urbanization.
The s p a t i a l l y variable nature of dynamic systems and the
complexity of the processes w i l l be increasingly reflected in
semi-distributed or distributed models. The t r a n s i t i o n from lumped
to d i s t r i b u t e d models i s provided by semi-distributed ( c e l l )
approaches which hold the promise of combining s p a t i a l
representation with the ease of calculation associated with lumped
models. Likely developments are in the direction of compartmental
models, although extensive research i s required concerning the
d e t a i l s of some of the compartments, such as évapotranspiration and
i n t e r c e p t i o n "losses". This cannot be done without the inclusion of
plant physiology. Coupling of compartments i s l i k e l y to give r i s e
to the analyses of error propagation, s e n s i t i v i t y and observability
of different model s t r u c t u r e s .
Increasing use of two-dimensional modelling i s foreseen, in for
instance:
- two-dimensional sheet flow models (two-dimensional cascades or
c e l l s , use of f i n i t e elements/differences),
- coupling of two-dimensional runoff models with d i g i t a l t e r r a i n
models,
- two-dimensional stochastic distributed catchment
modelling/filtering and estimation of d i r e c t l y unmeasurable (but
from the model point of view, observable) internal s t a t e s , including
uncertainty a n a l y s i s .
Spatial aggregation of data i s one of the weakest procedures of
c l a s s i c a l hydrology. This i s so because the large s p a t i a l
v a r i a b i i l i t y of natural hydrological processes and c h a r a c t e r i s t i c s
12
Andras Szôllôsi-Nagy et al.
has to be t a k e n i n t o account i n the i n t e g r a t i n g p r o c e s s e s with known
m i c r o s c a l e l a w s . The c r i t i c a l p o i n t s a r e the r e p r e s e n t a t i v e n e s s of
t h e measurements ( o b s e r v a t i o n s ) and the homogeneity of systems and
processes.
High technology i n n o v a t i o n s (Remote s e n s i n g and d a t a
t r a n s m i s s i o n ) combined w i t h p a t t e r n r e c o g n i t i o n and d a t a p r o c e s s i n g
w i l l allow t h e c h a r a c t e r i s t i c s of an a r e a r a t h e r t h a n a p o i n t to be
e v a l u a t e d . That i s t o say, t o t a l s and averages w i l l be a v a i l a b l e
and h y d r o l o g i s t s w i l l be r e l e a s e d from the need to u n d e r t a k e
u n r e l i a b l e a v e r a g i n g of a few p o i n t measurements.
I t seems probable t h a t the avenue provided by t h e i n t r o d u c t i o n of
the geomorphological u n i t hydrograph approach w i l l be developed i n
f u t u r e . Geomorphoclimatic approaches may h e l p i n the d e t e r m i n a t i o n
of parameters of p h y s i c a l l y - b a s e d s t o c h a s t i c compartmental models.
AfTHEOPOGEHC IMPACTS
Understanding and p r e d i c t i o n of the h y d r o l o g i c a l consequences of
anthropogenic impacts w i l l remain i n the f o r e f r o n t of i n t e r e s t f o r
s u r f a c e w a t e r h y d r o l o g i s t s . This m u l t i f a c e t e d problem embraces both
d i r e c t and i n d i r e c t human i n t e r v e n t i o n s i n h y d r o l o g i c a l regime. The
former group comprises such purposeful or i n c i d e n t a l m o d i f i c a t i o n s
a s u r b a n i z a t i o n , road c o n s t r u c t i o n i n c l u d i n g a c c e s s roads f o r
l o g g i n g , c h a n n e l i z a t i o n , r i v e r r e g u l a t i o n and l a n d - u s e and
v e g e t a t i o n changes i n c l u d i n g o v e r g r a z i n g and w i l d f i r e . Some l a r g e
s c a l e i m p a c t s , e . g . i n t e r r e g i o n a l water t r a n s f e r s and d r a i n a g e of
l a r g e swamps, may induce d i s t u r b a n c e s which p r o p a g a t e t o d i s t a n t
r e g i o n s v i a atmospheric dynamics. Major p r o j e c t s with the p o t e n t i a l
t o a l t e r s u b s t a n t i a l l y h y d r o l o g i c a l p r o c e s s e s should be monitored
s i n c e t h i s could f i l l gaps i n our u n d e r s t a n d i n g .
There a r e a l s o man-induced p r o c e s s e s , which do not d i r e c t l y
i n t e r v e n e i n the h y d r o l o g i c a l c y c l e , b u t can have s i g n i f i c a n t , and
t y p i c a l l y cumulative and l a g g e d , e f f e c t s on v a r i a b l e s and parameters
of s u r f a c e w a t e r h y d r o l o g y . The main e f f e c t i n t h i s c a t e g o r y i s t h e
i n c r e a s e i n C0 ? e m i s s i o n , which has been caused by t h e burning of
f o s s i l f u e l s and a r e d u c t i o n i n the a r e a of t r o p i c a l f o r e s t s and has
led to atmospheric warming v i a the greenhouse e f f e c t .
Likely
h y d r o l o g i c a l f a c t o r s t h a t must be taken i n t o account a r e changes i n
mean v a l u e s , v a r i a b i l i t y and temporal d i s t r i b u t i o n of p r e c i p i t a t i o n ,
runoff, é v a p o t r a n s p i r a t i o n , s o i l m o i s t u r e and drought. The p r e s e n t
o v e r a l l u n d e r s t a n d i n g of g l o b a l c l i m a t i c change and of i t s impact on
w a t e r r e s o u r c e s i s not s a t i s f a c t o r y .
The warm-up e f f e c t induced by
t h e i n c r e a s e of C0„ e m i s s i o n would cause i n c r e a s e d e v a p o r a t i o n , b u t
on t h e o t h e r hand i t would a l s o cause a c c e l e r a t e d growth of
v e g e t a t i o n and an i n c r e a s e i n p l a n t r e s i s t a n c e to d r o u g h t s through
more e f f e c t i v e w a t e r u s e . I n o r d e r to determine a n t h r o p o g e n i c
e f f e c t s p r o p e r l y , m o n i t o r i n g of b a s e l i n e v a l u e s i s r e q u i r e d .
The h y d r o l o g i c a l impact of acid r a i n w i l l a l s o be i n t e n s i v e l y
s t u d i e d . Apart from w a t e r q u a l i t y a s p e c t s , the p r o c e s s e s of
changing i n t e r c e p t i o n s t o r a g e and é v a p o t r a n s p i r a t i o n w i t h
a c i d i f i c a t i o n w i l l come under f u r t h e r i n v e s t i g a t i o n and t h i s can
lead to the r e - e s t i m a t i o n of design f l o o d s i n s m a l l e r catchments
Surface water hydrology
13
subject to acid r a i n and snow.
DEFICIESCIES OF STATISTICAL AID STOCHASTIC HYDEOLOGI
S t a t i s t i c a l and stochastic hydrology as a whole needs
reconsideration. We have probably reached the end of pure
s t a t i s t i c a l and stochastic hydrology, and the weakness of the
present s t a t e - o f - t h e - a r t will be increasingly recognized. I t should
be understood, for instance, t h a t standards or guidelines demanding
the use of some p a r t i c u l a r p . d . f . ' s for flood frequency analysis
have been introduced for the sake of uniformity rather than as a
proof of our knowledge. A solid physical explanation behind
s t o c h a s t i c hydrological phenomena i s s t i l l lacking.
Although the N-year flood i s one of the principal pieces of
information required from surface water hydrology by p r a c t i t i o n e r s ,
i t s d e r i v a t i o n is not supported by a credible procedure. One of the
weaknesses of this approach i s the impossibility to assess a 100- or
1000-year flood based on some 30 years of data. Attempts to assign
physical meaning to extreme value s t a t i s t i c s cannot be regarded as
being f u l l y successful and this problem i s l i k e l y to remain u n t i l
the end of this century, despite the fact t h a t a dozen or more years
of observations will have been accumulated by t h i s time. However,
even i f time series of data covering hundreds of years were
a v a i l a b l e , the present statement of the problem presumes
s t a t i o n a r i t y and homogeneity in flood occurrence. Evidence shows
t h a t these assumptions are not r e a l i s t i c since hydrological
processes are obviously nonstationary, being characterised by
n a t u r a l and anthropogenic p e r i o d i c i t i e s , trends and sudden changes,
and are c l e a r l y inhomogeneous with large floods being caused by a
number of factors. I t seems necessary to progress in the use of
nonstationary stochastic processes. I t i s not enough to account for
n o n s t a t i o n a r i t y in the mean since i t i s also necessary to consider
n o n s t a t i o n a r i t y in higher moments. I t i s also desirable to s t r i v e
towards the derivation of the d i s t r i b u t i o n of extreme value
s t a t i s t i c s (N-year floods) by taking into account the physics of the
process ( e . g . via physically-sound stochastic d i f f e r e n t i a l
equations).
OTHER COMSIDEBATIOIS
Apart from the topics elaborated i n more d e t a i l above, there are
several further specific problems relevant to surface water
hydrology that may d i r e c t development of this subarea.
For example, although in some regions and countries water balance
data a r e r e l a t i v e l y well known, t h i s i s not the case on the global
s c a l e . The assessment of volumes of water on the surface of the
Earth and underground i s neither accurate nor r e l i a b l e . Assessment
of global water balance has hitherto r e l i e d on r e l a t i v e l y crude
accounts of the oceanic phase of the hydrological cycle and, i n
p a r t i c u l a r , evaporation from oceans and p r e c i p i t a t i o n over ocean
s u r f a c e s . Even the net flow of water from land to ocean has not yet
14
Andras Szôllôsi-Nagy et al.
been precisely evaluated.
There is a need for a more precise evaluation of water balances
over areas of different scale (regions, countries, continents,
Earth) and for better estimation of fresh water reserves, that are
at present often inaccurately known, especially in less developed
countries.
Emergence of global hydrology offers challenging studies of
"teleconnections", i.e. high correlations between values of
hydrological variables observed in remote regions, with account of
transport time lags. For instance, intensity of Indian summer
monsoon rainfall and snow cover over Eurasia in the preceding winter
are strongly negatively correlated.
The necessary tools in the challenging research area of global
hydrology are general circulation models (GCM) and tracer
experiments allowing the elements of the global hydrological cycle
to be evaluated. Global scale numerical models should take into
account the interrelated physical, chemical and biological systems
of the Earth, i.e. interactions between hydrosphere, lithosphère,
atmosphere and biosphere.
There is a clear need to break down barriers between different
subareas of hydrology and between hydrology and other sciences.
"Green" hydrology provides examples of specific problems which
require interdisciplinary efforts for their solution. In order to
understand, assess and predict plant water use, it is necessary to
establish the transfer function which links water supply in terms of
volumes and timing, for example in irrigation practice and biomass
production.
The understanding of hydrological processes and their effective
management requires that surface water be studied in conjunction
with other processes. For example, further studies are needed in
order to understand interactions between surface and groundwater,
between the quantity and quality of surface water and also the links
between water and sediment flow. All three of these examples are
problems extending beyond the domain of surface water hydrology in
the traditional sense.
A catchment may be studied in a number of different contexts, and
the arsenal of available hydrological methods should contain
techniques applicable to catchments having different characteristics
such as scale, homogeneity, physiographic conditions, morphology,
climate, land use and so on.
It is essential to assess the accuracy and reliability of both
theoretical and practical methods of hydrology. This is possible,
however, only to a limited extent, which is connected with
identification of false oversimplified assumptions and their
consequences. It is not enough to supply numbers since it is
important to point out the credibility of these numbers so that the
appropriate design measures can be taken and a realistic safety
margin established.
Lake hydrology will probably turn further towards the application
of hydrodynamic models, particularly in three-dimensional models
which consider turbulence, analysis of water body/bottom sediment
interaction and more generally the connections with water quality,
including eutrophication problems. It must be mentioned, however,
Surface water hydrology 15
t h a t there are s t i l l problems in establishing lake water budgets,
p a r t i c u l a r l y on the evaporation s i d e .
A problem t h a t needs to be solved i n hydrological turbulence
studies i s the v e r i f i c a t i o n of predicted turbulent transport with
field data (both in terms of mean flow f i e l d and in the preservation
of spectral s t a t i s t i c s of fluctuating parameters).
River flow modelling i s a well developed area. One could expect
t h a t two-dimensional models will rapidly progress and w i l l include
the treatment of turbulence through the K-£ approach. This
p e r t a i n s , however, more to hydraulics than to hydrology and
applications of new models will most probably be applied in r i v e r
regulation and t r a i n i n g . As far as flood routing techniques and
r e a l time r i v e r flow models are concerned, developments are expected
through combination of deterministic and stochastic models and in
the treatment of backwater effects and l a t e r a l inflow i n
forecasting.