# Paleohydrology

Paleohydrology, or palaeohydrology, is the scientific study of the movement, distribution, and quality of water on Earth during previous periods of its history. The discipline uses indirect evidence to infer changes in deposition rates, the existence of flooding, changes in sea levels, changes in groundwater levels and the erosion of rocks. It also deals with alterations in the floral and faunal assemblages which have come about in previous periods because of changes in hydrology.

## Background

Past hydrological changes on our planet have had enormous effects on the environment. Over most of geologic time, the long-term mean sea level has been higher than today. Only at the Permian-Triassic boundary ~250 million years ago was the long-term mean sea level lower than today. Long term changes in the mean sea level are the result of changes in the oceanic crust, with a downward trend expected to continue in the very long term. Twenty thousand years ago, the sea level was 120 m (394 ft) lower than at present with vast amounts of water being locked up as ice; in the northern hemisphere, there was twenty times more ice than the present volume.
The climate of the Sahara, for example, has undergone enormous variations between wet and dry over the last few hundred thousand years, believed to be caused by long-term changes in the North African climate cycle that alters the path of the North African Monsoon. Paleohydrological studies of sediments in the Fazzan Basin in Libya show that humid conditions once prevailed there that were capable of creating a lake with a surface area of around 76,250 km$^2$ (29,400 sq mi). Before the abrupt desertification of North Africa about five thousand years ago, the Sahara was the home of Neolithic men and supported verdant vegetation and diverse wildlife.

## Methods

Paleohydrological study usually starts in the field with observations, measurements and the collection of samples; it continues with analysing the samples in the laboratory, recording the data, collating it, modelling systems, time-system analyses and eventually making inferences. A major step is the dating of the material. Methods here include using radioactive isotopes, considering the geological development of the area, the presence or absence of certain organisms and the identification of fossil pollen. Paleohydrology makes use of indirect methods that give an indication of the climatological conditions prevailing at the time.
Paleo hydrology uses methods that include using direct and indirect climatic data; these can be used to assess the variability in the hydrological cycle. Direct data encompasses measured and historical information, including streamflow records, flood occurrences, and drought events. In certain regions such as Egypt and China, direct data can extend back thousands of years, offering a rich historical perspective, while globally, it commonly spans approximately two centuries. Complementing direct data, indirect data—often referred to as proxy data—serves to extend climatic and hydrological insights. For instance, the analysis of tree rings allows for the reconstruction of past precipitation and temperature patterns, while deep-sea sediment cores contribute to predictions of long-term global temperatures. Proxy data is instrumental in providing evidence for prehistoric floods, with its traces commonly preserved in sedimentological deposits within streambeds and botanical evidence. This collective data is crucial for forecasting the frequency and magnitude of floods and droughts in specific geographic areas.

## Applications

The methodologies outlined in the methods section facilitate the determination of flood occurrences, magnitude, and ages. Through these techniques, paleoflood data can be extended back over thousands of years, enriching the precision of flood-frequency curves. This extended historical perspective is invaluable in contemporary flood-frequency analysis, significantly amplifying the effective length of the record. The incorporation of historical flood data enhances the analysis, offering a more comprehensive understanding of the dynamics and patterns involved in flooding events.
Paleohydrological data serves as a valuable tool in unraveling the climatic variability of the past. Evidence of climatic changes is seen in lake and ocean sediment, as well as in the mass balance of glaciers. Over the last 10,000 years, the climate has undergone significant fluctuations, impacting floods, droughts, and hydrologic patterns. Understanding this historical climatic variability is crucial for predicting future climate changes. Take, for instance, the Colorado River, a vital freshwater source for the southwestern United States. By analyzing data from past droughts, it becomes evident that recent climatic variability could potentially reduce streamflow by 35 percent. This knowledge is indispensable for informed future planning, ensuring water availability for the populations that depend on it.
Hydrological fluctuations are linked to the factors causing them, and paleohydrological data can be used to validate climate models. On the orbital time scale, paleohydrological data reflects variations in the Earth's orbit and the cycle of glacial periods and interglacials. For example, variations in the water level of Lake Lisan correlates with data showing temperature fluctuations collected from polar ice core samples. On a shorter time scale, minuscule climatic variations can have large hydrological effects as when excess rainwater entering the North Atlantic was linked with a serious drought in the eastern Mediterranean. The Little Ice Age in northern Europe was linked with drought in East Africa, heavy rains in the African lakes, and persistent El Niño–Southern Oscillation conditions in the Pacific.
Another application is in the quantification of erosion caused by rivers under differing climatological conditions. Increased erosion rates following deforestation, and pollution resulting from lead-mining activities by the Romans show up in lake sediments.