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Here we analyse geomorphic, pedologic and hydrologic processes of relevance to archaeological research on the pre-colonial irrigation system at Engaruka, Tanzania. Although archaeological studies have been carried out in Engaruka for several decades, geophysical processes have not been in focus, despite having the potential to contribute to the understanding of both the ancient irrigation system and the current land use in Engaruka. Geomorphology and pedology were explored through field surveys and mapping using air photos and satellite images. The effects of the flash-flood during the 1997–1998 El Niño were mapped and quantified in the field. Maximum flash-flood discharge in Engaruka River was estimated using Manning's Equation. The geomorphic study revealed that Engaruka is an environment where high-magnitude processes (debris flows, flash-floods, rock slides) dominate landform development. Low-magnitude processes (sheet-wash, wind erosion), transfer fine-textured sediment to low terrain, causing successive coarsening of soil texture in fields of the ancient irrigation system. Andisol occurrence in the dry environment is associated with current irrigation, indicating incremental improvement of arable land resulting from human land use. Andisols appear transient; discontinuation of irrigation has caused a reversion to Entisols, as water retention and structure deteriorate upon drying. A flash-flood during the 1997–1998 El Niño caused destruction of parts of the archaeological remains, with severe impact on current land use and settlements. Approximately 80 ha (4%) of the total area of the ancient remains were covered by debris flow deposits and alluvium, or scoured by new drainage lines of the water courses. The denudation caused by the single flash-flood is estimated to 9 mm in the catchment of Engaruka River. Manning's Equation yielded a least possible maximum discharge in Engaruka River, in the order of 270 m 3 /s, some 600–700 times higher than normal, dry season discharge. [ABSTRACT FROM AUTHOR]

A history of modern (1973-2015) surface area variability for Lake Basotu in north-central Tanzania has been reconstructed using archived Landsat images from the dry season between June and October. This record was compared to local weather data as well as larger scale weather patterns. The lake has been in a state of decline interrupted by major flood events since the beginning of the satellite record. From 1973 to 1997, the lake area was between 0.97 km² and 4.28 km². Lake extent abruptly increased to 13.86 km² in 1998, when a co-occurrence of El Niño and a positive Indian Ocean Dipole led to extensive flooding. It is hypothesized that local agricultural practices leading to soil erosion and subsequent basin sedimentation have most likely increased the sensitivity of Lake Basotu to climatic fluctuations. [ABSTRACT FROM AUTHOR]

Lake Duluti is a small, topographically closed crater lake located on the flanks of Mt Meru, northern Tanzania. Analyses of diatoms in three short sediment cores and four modern samples from Lake Duluti were used to infer past environmental changes. Pb and Cs activity profiles combined with AMS C dates provide the chronological framework. Weak agreement between the Pb and C records, together with dating uncertainty, precludes construction of precise age models. The modern diatom flora, from plankton and three periphytic habitats, is dominated by Aulacoseira ambigua (Grunow) Simonsen, Gomphonema parvulum (Kützing) Grunow and Nitzschia amphibia Grunow. All three cores display similar stratigraphic succession, but the relative ratio of habitats represented by the diatoms varies substantially between cores. Diatoms indicate that the oldest part of the record is characterized by relatively low lake level and swampy vegetation. In the late nineteenth or early twentieth century there was a rapid lake level rise and the swamp turned into an open-water lake. High lake levels have prevailed since that time. [ABSTRACT FROM AUTHOR]

Climate data from Empakaai Crater in northern Tanzania, covering the last 1200 years, are related to the establishment, development and decline of the ancient irrigation system at Engaruka. New dates for the system are linked to reconstructed climatic variations and historical data on long-distance and regional trade and migration patterns. A shift from a comparatively humid climate to drier conditions in the 1400s prompted the establishment of irrigated agriculture at Engaruka, and a flourishing long-distance trade increased its value as a water and food source for passing caravans. Once established, the land-use system at Engaruka was sufficiently resilient to survive and even intensify during much drier climate from c. 1500 to 1670 CE (Common Era) and during the decline of caravan trade between c. 1550 and 1750. The ancient land-use system probably reached its maximum extension during the humid conditions between 1670 and 1740, and was deserted in the early to mid 1800s, presumably as a result of the added effects of climate deterioration, the Maasai expansion, and change of livelihood strategies as agriculturalists became pastoralists. Towards the end of the 1800s irrigated agriculture was again established at Engaruka, in part driven by the transfer from pastoral to agricultural livelihoods caused by the Rinderpest. [ABSTRACT FROM AUTHOR]

Abstract: Fossil pollen and diatoms have been analyzed in a sediment sequence from a topographically closed crater lake in northern Tanzania (Lake Duluti), with the aim to reconstruct past changes in lake level and vegetation dynamics. The results provide a new paleoenvironmental record from equatorial Africa covering the period c. AD 1000 to AD 1800. Overall, the pollen and diatom records generate comparable stories of dry and wet periods. Dry conditions are inferred at c. AD 1040–1470, c. AD 1510–1640 and c. AD 1650–1670 with the lowest lake levels at c. AD 1260–1290 and AD 1600–1640. Wetter conditions occurred c. AD 1640–1650 and c. AD 1670–1790. The chronology is based on combined analyses of 210Pb activity and AMS 14C on bulk sediment, and a Bayesian model was applied to establish the age-depth relationship. The hydroclimatic record from Lake Duluti shows good correlation with several East African lakes in a centennial time perspective, although comparison of high frequency variability in the region is hampered by dating uncertainties. [Copyright &y& Elsevier]