palaeohydrology

palaeohydrology Palaeohydrology has been defined by Stanley Schumm as ‘the study of the quantity, quality and distribution of surface waters in the past’. This broad subject can be divided into three basic approaches: inferential palaeohydrology, classical palaeohydrology, and palaeohydrological modelling.Inferential palaeohydrology is the hydrological interpretation of sedimentary sequences or landforms, where a change in hydrological conditions is inferred but no attempt is made to estimate its magnitude. This is extremely common in the interpretation of fluvial sequences and lake sediments (inferring depth or volume changes) of all ages. It is also generally a prelude to more formal palaeohydrological work as well as the standard geomorphological method of linking changes in Earth surface processes and rates to changes in the environment such as climate and land use.Classical palaeohydrology is the estimation of past hydrological parameters from sediments or landforms. One approach is the estimation of palaeoflood discharges, in particular form slack water deposits at tributary junctions and on benches within reaches confined by bedrock (i.e. undeformable channel beds; Fig. 1). The flood slack-water sediments are then dated (e.g. by radiocarbon or luminescence, or by archaeology) and this makes it possible to calculate a return interval for the water level (stage), or flood discharge, if flow can be modelled (e.g. by using slope–area or step–backwater methods). The practical application of this methodology is to calculate to return intervals of floods far beyond the instrumented record. The second approach in classical palaeohydrology is the estimation of discharge from palaeochannel parameters. This is based upon the well-understood relationships between modern channels and discharge. The most obvious case is the estimation of bankfull discharge from palaeochannel capacity (A) and gradient, using one of the well-known flow equations, such as the Manning's or Darcy Weisbach for velocity (V), and the continuity equation (Q = VA) (see river channel hydraulics) There are many sources of potential error even when the palaeochannel is well preserved and of a sufficient length. An alternative is to use empirical relationships between channel parameters, such as width or meander wavelength, and a significant discharge (e.g. mean winter flow, or bankfull flow). This method was used by Stanley Schumm in his classic study of two generations of palaeochannels (prior and ancestral streams) of the Murrumbidgee on the Riverine Plain of Australia.

A third method is to estimate flow discharge and velocity from fluvial sediments and structures. Bedforms such as ripples and dunes can for example, indicate flow regime and depth. This approach has been used by John Allen in his classic studies of the Old Red Sandstone, and has been used to estimate the magnitude of the mega-floods associated with the catastrophic drainage of Lake Missoula at the end of the Last Glacial period in North America. In addition, the flow velocity can be estimated from the size of the clasts (particles) moved, or indeed not moved, by the flow, an approach known as ‘palaeohydraulics’. Victor Baker was one of the first to use this approach in his classic study of the alluvium at Golden, Colorado. There are, however, many problems with all these methods; defining channel width can be difficult, as can the estimation of channel gradient and the size of the largest entrained particle. Additional problems arise from the state of the channel bed, such as armouring, loose and over-loose bed conditions, and sediment supply. For these and other reasons, a realistic estimate of the level of accuracy attainable is probably only an order of magnitude.

Palaeohydrological modelling is a deductive process in which an estimate is made of the past water balance of an area from hydrometeorological estimates and catchment conditions. It is most obviously applied to modelling the hydrological balance of lakes, in order to estimate precipitation (P) using the standard water-balance equation (P = R + (E + T) + (DS + DG) ) from runoff (R), evaporation (E), transpiration (T), and changes in surface-water storage (S) and groundwater flux (G). There are, however, problems and there is considerable ‘noise’ in the records of lake levels (as held by various lake-level databases), principally derived from problems in estimating palaeo-depths and unknown groundwater fluxes. A similar approach has been used for river basins, an example being John Lockwood's estimate that a small catchment in central England could, in the mid-Holocene, have had a 10 per cent lower discharge despite 10 per cent more rainfall as a result of the greater interception and evapotranspiration of a complete forest cover. Palaeohydrological modelling, whether from lakes or river basins provides a bridge between the scale of global climate models and the sedimentary section or borehole record.

There are several other approaches to estimating past hydrometeorological conditions that are closely related to palaeohydrology. One is the analysis and climate interpretation of historical hydrological records, generally of river levels (historical hydrology). A second is the reconstruction of precipitation and stream flow from tree rings (dendroclimatology and dendrohydrology). What all these methods have in common is that they attempt to provide quantitative estimates of past hydrological conditions, which can at least stand as testable hypotheses. The palaeohydrological approach has altered our ideas about hydrological stationarity (the assumption of a constant relationship between flood magnitude and frequency), enabling us to measure, as opposed to estimating by extrapolation, the magnitude of rare events. Palaeohydrological studies have relevance to the flood and drought hazards faced by people in the past, today, and in the future.

A. G. Brown

Bibliography

Allen, J. R. L. (1984) Sedimentary structures: their characteristic and physical basis. Elsevier Science Publishers, Amsterdam.
Baker, V. R.,, Kochel, R. C.,, and and Patton, P. C. (1988) Flood geomorphology. John Wiley and Sons, New York.
Benito, G.,, Baker, V. R.,, and and Gregory, K. J. (1998) Palaeohydrology and environmental change. John Wiley and Sons, Chichester.
Brown, A. G. (1977) Alluvial Environments: Geoarchaeology and Environmental Change. Cambridge University Press.