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agro-pastoral landscapes along Wadi Sana (Yemen)
Journal of Arid Environments 86 (2012) 131e138
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Journal of Arid Environments journal homepage: www.elsevier.com/locate/jaridenv
Hydro-geospatial analysis of ancient pastoral/agro-pastoral landscapes along Wadi Sana (Yemen) M.J. Harrower a, *, E.A. Oches b, J. McCorriston c a
Department of Near Eastern Studies, Johns Hopkins University, 3400 N. Charles St., Gilman Hall 113, Baltimore, MD 21218, USA Department of Natural and Applied Sciences, Bentley University, USA c Department of Anthropology, The Ohio State University, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 February 2011 Received in revised form 31 October 2011 Accepted 24 November 2011 Available online 15 December 2011
In arid Southwest Arabia spatio-temporal patterning of land and water resources substantially shaped the lives and livelihoods of ancient peoples. This paper examines mid Holocene human subsistence and hydrology along the Wadi Sana drainage, including increased aridity, an associated shift from alluvial aggradation to incision, and its role the appearance of irrigation strategies that supplemented herding economies. Geospatial analyses indicate that (rather than solely the result of increased aridity) the onset of incision along middle Wadi Sana was most plausibly a function of heightened inter and intra-annual precipitation variability. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Arabia Archaeology Geographic information systems Hydrology Irrigation Remote sensing
1. Introduction Ancient pastoral and agro-pastoral societies of southern Arabia devised a range of unique traditions and strategies to survive and thrive in rugged, hyper-arid tropical environments. Indeed, lifeways of ancient south Arabian populations differed substantially from those of archaeologically more well-known areas of the ancient Near East (including The Levant, northern Mesopotamia, and Anatolia) where temperate environments contributed to a far less circumscribed range of subsistence opportunities. Notably, in Southwest Arabia a phase of independent pastoralism was followed around 5500 cal. BP by the appearance of irrigation and integration of agricultural and pastoral strategies. This paper adapts geospatial technologies including satellite imagery and HEC-RAS (Hydrologic Engineering Center’s River Analysis System) hydrological modeling software to examine connections between ancient human subsistence and the paleohydrology of the Wadi Sana watershed of Hadramawt Governate, Yemen. As part of the larger Roots of Agriculture in Southern Arabia (RASA) Research Project initiated in 1996, this report draws in particular on evidence
* Corresponding author. Tel.: þ1 410 516 7500. E-mail address: [email protected] (M.J. Harrower). 0140-1963/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jaridenv.2011.11.020
collected during fieldwork in 2000, 2004, 2005, 2007 and 2008 (McCorriston et al., 2002, 2005, 2011), including radiocarbon assays from alluvium, GPS cross-sections of Wadi Sana, and Digital Elevation Models (DEM) derived from ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) and SRTM (Shuttle Radar Topography Mission) spaceborne imagery. We examine a welldocumented and radiocarbon dated ca. 5200 cal BP shift from aggradation to incision along Wadi Sana and conclude that this change was not solely the result of decreased annual precipitation but was more proximately a function of a concomitant increase in inter and intra-annual precipitation variability (see Dewar and Wallis, 1999; Lézine et al., 2010; Sanlaville, 1992) with attendant shorter duration, high-volume discharges. Moreover, we propose that increasing aridity and resultant plant food scarcity contributed to the emergence of agro-pastoralism in the 6th millennium BP in which small-scale irrigation of drying and incising alluvial sediments supplemented pre-existing herding economies. 2. Context, methods and results A wide variety of environmental factors variable at differing spatio-temporal scales including structural geology, climate, geomorphology and hydrology influenced the range of subsistence opportunities available across ancient southern Arabia. The bedrock of this research area of eastern Yemen’s Hadramawt Governate is an
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uplifted Paleocene-Eocene carbonate platform, exposing at the surface the Umm ar-Radhuma Formation (Beydoun, 1964). This widespread limestone plateau is dissected by Wadi Hadramawt (renamed Wadi Masila east of the city of Tarim) and its tributaries. In northern reaches Umm ar-Rudhuma limestone is overlain by the lower Eocene Jeza Formation limestone; both units are deeply incised, creating a stepped dendritic network with scattered buttes of thin bedded Jeza shale-limestone above the plateau surface and up to three sets of paired terraces incised during episodic postMiocene platform uplift. Wadi Sana, one of the major northflowing tributaries of Wadi Hadramawt/Masila, is formed by a teardrop-shaped watershed at that covers nearly 4000 sq km. South of the town of Ghayl bin Yumain a large sub-basin converges to form the headwaters of Wadi Sana’s main channel (Fig. 1). Throughout the western portion of this sub-basin (hereafter referred to as the Ghayl bin Yumain basin) sizeable expanses of alluvium, gypsum, and marl deposits form a broad flat plain that includes remnants of the Lower-Middle Eocene Rus formation. Presently and in the past the Ghayl bin Yumain basin has been the primary source for alluvial sediments in Wadi Sana. Changes in sediment supply and fluvial transport capacity from this basin, related to variations in regional precipitation and surface-water runoff, were of critical importance in regulating patterns of alluvial deposition and erosion along the middle and lower reaches of Wadi Sana’s main channel (Anderson, 2007). Precipitation across eastern Yemen is primarily a function of southwest monsoon systems that carry moisture from the Indian Ocean during the late spring and summer. These systems are largely controlled by seasonal and longer interval shifts in the position of the Intertropical Convergence Zone (ITCZ), a prominent atmospheric boundary where subtropical and equatorial air masses converge contributing to markedly higher rainfall. Today, limited summer monsoon precipitation (Fisher and Membery, 1998; Sanlaville, 1992) falls on deeply dissected, scree-covered limestone terrain, leading to flashy, short-duration, large volume discharges in valley bottoms that incise and erode alluvial sediments. However, very different
precipitation and runoff conditions must have prevailed during the early Holocene, when precipitation was more frequent and plentiful, and alluvium was aggrading along middle Wadi Sana. For more than 100,000 years these rugged, arid tropical environments were home to human populations who survived as hunters and gatherers (Rose, 2010). In the early Holocene evidence of domesticated animals first appears in the region, including at Wadi Sana’s Manayzah rockshelter (Crassard et al., 2006; Martin et al., 2009; McCorriston and Martin, 2009). Pastoralists who raised sheep, goat and cattle would have relied on moist bottomlands along middle Wadi Sana to sustain their herds, particularly cattle which would have required habitats capable of producing copious grassy forage. A ring of more than 40 cattle skulls discovered by RASA in 2005, excavated and dated to ca. 6300 cal BP reveals the profound economic and social importance of cattle, which nevertheless waned after precipitation declined during the middle Holocene (McCorriston, 2011; McCorriston et al., in press). In concert with declining precipitation near 5200 cal BP, residents of the area began building small irrigation structures to divert water onto arable alluvium (Harrower, 2008a,b). Although we do not know for certain what plants were watered in the small fields they created (Harrower, 2006; McCorriston, 2006), we do know that geomorphology and hydrology of Wadi Sana played an important role in their lives, decisions and choices. 2.1. Archaeological/geological fieldwork Roots of Agriculture in Southern Arabia (RASA) Project fieldwork in winter 2000 and 2004 involved geological and archaeological survey along middle Wadi Sana, focusing in particular on alluvial deposits, as they preserve considerable early to middle Holocene archaeological remains and offer the most promising contexts for preservation of stratified archaeological deposits suitable for excavation. Considerable interest centered on the chronology and geomorphology responsible for alluvial deposition and erosion. Geological inspection and sampling of alluvium, predominantly
Fig. 1. The Wadi Sana watershed of Hadramawt Governate, Yemen. The Ghayl bin Yumain Sub-Basin and locations of 17 GPS cross-sections along middle Wadi Sana indicated (three cross-sections along Wadi Shumlya excluded).
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coarse silt plus fine to medium grain sand (Anderson, 2007; Oches et al., 2001), and both intensive and extensive archaeological survey along Wadi Sana (McCorriston et al., 2005) began to reveal a pattern of early Holocene aggradation followed by a mid-Holocene shift to incision. Of the 39 radiocarbon assays we measured on charcoal buried in alluvium along Wadi Sana, all but two anomalous samples yield ages older than 5144 cal yr BP, indicating that aggradation largely ceased near this time (Harrower, 2006: 106). This pattern is consistent with marine and terrestrial records for the region that show an interval of increased precipitation during the early Holocene, followed by a prominent shift toward aridly near 5500 cal yr BP (Fleitmann et al., 2003, 2007; Lézine et al., 1998, 2007, 2010; Parker et al., 2006; Sirocko et al., 1993; Wilkinson, 1997, 2005). A wide range of natural or cultural factors could potentially be responsible for the shift from aggradation to incision along Wadi Sana, including changes in channel morphology or slope, sediment supply or grain size, water volume, timing or variability of precipitation and runoff, grazing pressure, deforestation, or human manipulation of surface sediment or water resources. 2.2. Topographic and landform analyses Hydro-geospatial analysis can substantially clarify hydrological and geomorphologic conditions relevant to pastoral and agropastoral livelihoods. Most importantly, movements of pastoralists who are often heavily dependent on their herds are closely connected with the availability of water and forage. During the early Holocene water would have been readily available from springs and spring-fed wetland ponds along Wadi Sana, while forage would have required sediments suitable as grass habitat. Similarly later agro-pastoral populations were also dependent on water flow patterns and the spatial distribution of sediments that could be irrigated. Wadi Sana offered relatively attractive opportunities, albeit opportunities that changed considerably through time. Through geomorphic survey and satellite imagery analysis we defined seven landform types: bedrock slopes, scree slopes, plateaus, bedrock terraces, wadi channels, wadi silts, and gravel terraces (Harrower, 2008a; Harrower et al., 2002). The first five of these landforms rarely (if ever) offered suitable habitats for grazing and probably never offered opportunities for cultivation. The final two types, ‘wadi silts’ and ‘gravel terraces’ represent sedimentary deposits that infilled Wadi Sana during the terminal Pleistocene and early Holocene. Wadi silts are a combination of eolian silts and alluvial silty sand that accumulated episodically as atmospheric dust and overbank alluvium deposition. Gravel terrace sediments are pebble, cobble, and boulder gravels deposited in former channels as high-energy bedload sediments. Beginning during the midHolocene incision and erosion left patches of these sedimentary deposits that offer the only suitable grazing and cultivation surfaces available. 2.3. Models of pastoral grazing and agro-pastoral irrigation Two different Digital Elevation Model (DEM) datasets, SRTM and ASTER, were used to analyze terrain conditions relevant to ancient pastoral grazing and agro-pastoral irrigation. First, six separate (one by one degree) scenes of SRTM data were merged to create a DEM consisting of 90 m 90 m grid cells for the entirely of modern day Yemen. Using ArcHydro tools for ArcGIS (Maidment, 2002) a water flow accumulation layer for the same area was then generated by modeling flow from higher to lower elevation DEM grid cells (Harrower, 2009). Among a variety of other insights, this layer reveals that peak flow accumulation at the mouth of Wadi Sana (484,559 grid cells or 3924 sq km) falls above the 99.9th percentile of flow accumulation grid cells in Yemen (the maximum accumulation
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value of 10,271,700 grid cells or 83,201 sq km occurs at the mouth of Wadi Masila as it enters the Indian Ocean near Sayhut). To determine the extent of areas that could have offered suitable grazing territory, the aforementioned SRTM flow accumulation layer was used to produce a drainage network for areas receiving runoff from more than 5000 cells (40.5 sq km). While somewhat subjective, this threshold was selected based on the rule-of-thumb 5000 cell recommendation of Maidment (2002: 73) and first-hand familiarly with the landscape. The resultant dendritic drainage network substantially assists in defining areas of Wadi Sana and its tributaries that could have received sufficient runoff to sustain grasslands. Noticeable, however, are vast differences in the areal extent of bottomlands between bedrock along this network. That is, higher order portions of the network have far wider channel and hence more area available for alluvium to accumulate. To account for this, the network was buffered (i.e. widened) according to Strahler stream order (1st order ¼ 100 m, 2nd ¼ 250 m, 3rd ¼ 400 m, 4th ¼ 550 m) delineating a 7920 ha area of bottomlands along middle and lower Wadi Sana. Overlaying this buffer area on a landform classification layer produced from ASTER satellite imagery (Harrower, 2006, 2008a) shows that wadi silt and gravel terrace landforms (areas that most plausibly could have sustained grass) today comprise approximately 1767 hectares postincision (McCorriston et al., in press), and during the early Holocene (pre-incision) these landforms would have covered a much wider area offering considerable grazing territories for cattle and other wild and domestic ruminants. Alluvial sediments that were initially important for pastoral grazing later become crucial for agro-pastoral irrigation. Previously reported results modeled the distribution of 174 ancient irrigation structures along Wadi Sana identified by archaeological survey (Harrower, 2008a,b, 2010). These analyses employed a higher resolution 15 m DEM extracted from ASTER imagery and show clear associations between locations of irrigation structures, water flow accumulation, and alluvial sediments. Incipient irrigators selected low to moderate flow accumulation areas where water could be slowed and/or diverted onto small plots of wadi silts that served as small 0.5 to 1 hectare fields (Harrower, 2008a). 2.4. Channel cross-section modeling with HEC-RAS To further examine water flow patterns and evaluate potential causes of the middle Holocene shift from aggradation to incision, HEC-RAS software was used to analyze GPS cross-section transects of Wadi Sana (Figs. 2 and 3). During fieldwork in 2004 we measured elevations along twenty linear cross-sections of Wadi Sana using a Trimble Pathfinder ProXRS differential GPS backpack. Given difficulties with noise in the data, in 2008 some cross-sections (T4, T5, T6, T7, T8, T15, T16, T17, T18, T19) were re-measured with a more accurate Trimble 5700 kinematic GPS backpack. Since both GPS receivers were simply measuring a location every second regardless of whether the operator needed to make a detour (e.g., to climb a steep rise or more around a group of acacia trees), all GPS locations were adjusted to reflect elevation along linear cross-sections using the Point Distance function of ArcGIS. These twenty transects, located along a 52.3 km stretch of middle Wadi Sana (Figs. 2 and 3), shed considerable new light on the aggradation/incision transition. Subcritical steady flow modeling was performed using HEC-RAS software (US Army Corps of Engineers, 2010). Boundary condition estimates included a 600 m water surface elevation at the most downstream Wadi Sana cross-section (T26) along with discharge estimates of 700 m3/s and 300 m3/s at the most upstream crosssections of Wadi Sana (T2) and Wadi Shumlya (T4) respectively. Subcritical steady flow analysis run in HEC-RAS then generated water surface elevations along the length of Wadi Sana and Shumlya
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Fig. 2. GPS cross-section T15 shown on a Quickbird satellite image and in a digital photograph as an example of our methods. From foreground to background five different landform types, wadi silts, wadi channel, bedrock slope, plateau and scree slope can be seen in the photograph. After mid-Holocene incision, check dams built in small channels between wadi silt banks and bedrock uplands (including the channel on the extreme right side of the cross-section) were used for irrigation.
as depicted in Fig. 3. Most alluvial sediments were deposited during the early Holocene along middle Wadi Sana where a substantial widening of the area between bedrock uplands and flattening of the channel contributed to reduced transport capacity (i.e. T15, T16, T17, T8, T18, T19). In simple terms, aggradation/incision can be modeled as a balance between sediment supply (or yield) and transport capacity. If input results in a transport capacity that is less than the sediment supply, a channel will tend to aggrade. Alternatively, if input results in a transport capacity that is greater than sediment supply, a channel will tend to erode. As quantified in equation (1), where qs is sediment supply or yield (at time one, qs1, and time two, qs2), q is discharge, S is channel slope, and D is sediment caliber (Clark and Wilcock, 2000), sediment caliber increases promote aggradation, while discharge and slope increases promote incision.
qs1 ¼ qs2
q1 q2
2 2 3=2 S1 D2 S2 D1
(1)
These considerations, along with HEC-RAS analyses, allow us to rule out a number of potential causes for the mid-Holocene shift to incision. Along its length Wadi Sana exhibits no major slope
changes, running a roughly 3.4% grade from an elevation of 774 m to 597 m over 52.3 km (Fig. 4). Similarly, sediment caliber is unlikely to have changed significantly, as the Ghayl bin Yumain basin remained the primary source of alluvial sediment throughout the Holocene (Anderson, 2007). Therefore S and D parts of the equation factor to 1, and manipulating q heuristically shows that a reduction in discharge (as a result of decreasing mean annual precipitation) would lead to reduced transport capacity and thus likely aggradation rather than degradation. Concordantly, one would expect that the mid-Holocene decrease in annual precipitation identified in numerous independent paleoclimatic records (e.g. Fleitmann et al., 2007; Lézine et al., 2010; Parker et al., 2006) would have promoted aggradation rather than incision. Notably, a drop to ½ the original early Holocene discharge would be capable of transporting only about ¼ of the original amount of sediment (see Wilson, 1973). A decrease in precipitation alone is therefore unlikely to have caused the profound shift from aggradation to incision we documented during fieldwork. The most plausible alternative (that is left unaccounted for if one considers Eq. (1) only in terms of unidirectional increases or decreases in precipitation) is oscillating inter and intra-annual precipitation changes. It has long been known that annual precipitation variability typically increases
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Fig. 3. GPS cross-sections of Wadi Sana from upstream to downstream. All cross-sections plotted on a common scale using HEC-RAS software (note: scale of X and Y axes differ, three cross-sections of Wadi Shumlya excluded). The shaded area represents estimated subcritical flow using a 600 m water surface elevation for the most downstream crosssection (T26) and discharge estimates of 700 m3/s and 300 m3/s for the most upstream cross-sections of Wadi Sana (T2) and Wadi Shumlya (T4) respectively. A substantial widening of the area between bedrock uplands along middle Wadi Sana evident from T15, T16, T17, T8, T18 to T19 led to reduced transport capacity and resultant early Holocene alluvial deposition in that area.
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Fig. 3. (continued).
Wad_Sana Reach2
Wadi_Sana Reach1
W a d i _ S h u m l y a
800
750
R e a c h 1
Legend Ground
Elevation (m)
700
650
600
550 0
10000
20000
30000
40000
50000
60000
Main Channel Distance (m)
Fig. 4. Middle Wadi Sana profile plot generated by HEC-RAS software. Plot shows channel slope along Wadi Sana and Wadi Shumlya as measured by 20 GPS cross-sections.
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as mean precipitation declines, particularly in the tropics (Dewar and Wallis, 1999). Thus we propose that it was not solely a decline in precipitation, but a concomitant increase in precipitation variability that is most proximately responsible for the shift from aggradation to incision along middle Wadi Sana. Indeed, if a change in rainfall regimes was responsible for incision, then either precipitation events were shorter and more intense or regional models are not correctly identifying localized increases in mean annual precipitation for Wadi Sana; given the comparative wealth of data for a mid-Holocene shift to aridity we consider the former far more likely. 3. Conclusions: hydro-geospatial changes and pastoral/agro-pastoral landscapes The spatio-temporal distribution of water flow across ancient south Arabian landscapes was of central importance to ancient herders and transhumant farmers. New geospatial datasets and technologies offer a wealth of analytical opportunities. We have used GIS modeling tools to shed new light on the mid-Holocene shift from aggradation to incision along ancient Wadi Sana. Though in some previous publications we interpreted this shift simply as the result of decreased precipitation, present analyses suggest that it was not only a decrease in annual water volume, but a likely increase in the seasonal and annual variability of precipitation, including episodic high discharge events that would have been most important in instigating the shift to erosion and incision. Intervening low flow conditions may have been unable to mobilize and transport even fine-grained sediment from the Ghayl bin Yumain basin (Anderson, 2007) and therefore do not appear to have offset the erosive effect of periodic high volume flows. The long-term dynamics of water and sediment along Wadi Sana held considerable importance for ancient residents of the area. Wet bottomlands would have been crucial to families with cattle, and the drying and erosion of those areas would have contributed to changing lifeways, including the eventual disappearance of cattle (in favor of sheep, goats and eventually camels), along with experimentation and reliance on small-scale irrigation. Alluvial sediments were similarly important to agro-pastoralists, and as middle Wadi Sana began incising and wadi silt sediments became stranded high ground irrigation strategies were devised to ensure that remnant sediments surfaces were sufficiently watered. Though we certainly would not suggest that environmental changes determined trajectories of human history, they undoubtedly did play a substantive role in shaping the range and expression of different options and geospatial technologies hold a promising future in explicating ancient choices and human-environment relationships. Acknowledgements This paper greatly benefited from the combined efforts of numerous students and scholars who contributed to the Roots of Agriculture in Southern Arabia (RASA) research project, most notably Joshua Anderson whose thesis laid the groundwork for these analyses. We are particularly grateful to the Yemeni General Organization for Antiquities and Museums including Abdullah Ba Wazir, Abdalaziz Bin ’Aqil, Khalid Ba Dhofary, Ietha Al-Amary, and Abdalbaset Noman. We also thank Canadian Nexen Petroleum, particularly Gregor Mawhinney, Alan Brindley, Kevin Tracy, Rick Jensen, Dave Smith, and Kevin Marlow for enormous logistical support over nearly a decade. Thanks also to Peter Wilcock who offered comments and advice that proved central to the analyses of this paper.
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Journal of Arid Environments journal homepage: www.elsevier.com/locate/jaridenv
Hydro-geospatial analysis of ancient pastoral/agro-pastoral landscapes along Wadi Sana (Yemen) M.J. Harrower a, *, E.A. Oches b, J. McCorriston c a
Department of Near Eastern Studies, Johns Hopkins University, 3400 N. Charles St., Gilman Hall 113, Baltimore, MD 21218, USA Department of Natural and Applied Sciences, Bentley University, USA c Department of Anthropology, The Ohio State University, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 February 2011 Received in revised form 31 October 2011 Accepted 24 November 2011 Available online 15 December 2011
In arid Southwest Arabia spatio-temporal patterning of land and water resources substantially shaped the lives and livelihoods of ancient peoples. This paper examines mid Holocene human subsistence and hydrology along the Wadi Sana drainage, including increased aridity, an associated shift from alluvial aggradation to incision, and its role the appearance of irrigation strategies that supplemented herding economies. Geospatial analyses indicate that (rather than solely the result of increased aridity) the onset of incision along middle Wadi Sana was most plausibly a function of heightened inter and intra-annual precipitation variability. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Arabia Archaeology Geographic information systems Hydrology Irrigation Remote sensing
1. Introduction Ancient pastoral and agro-pastoral societies of southern Arabia devised a range of unique traditions and strategies to survive and thrive in rugged, hyper-arid tropical environments. Indeed, lifeways of ancient south Arabian populations differed substantially from those of archaeologically more well-known areas of the ancient Near East (including The Levant, northern Mesopotamia, and Anatolia) where temperate environments contributed to a far less circumscribed range of subsistence opportunities. Notably, in Southwest Arabia a phase of independent pastoralism was followed around 5500 cal. BP by the appearance of irrigation and integration of agricultural and pastoral strategies. This paper adapts geospatial technologies including satellite imagery and HEC-RAS (Hydrologic Engineering Center’s River Analysis System) hydrological modeling software to examine connections between ancient human subsistence and the paleohydrology of the Wadi Sana watershed of Hadramawt Governate, Yemen. As part of the larger Roots of Agriculture in Southern Arabia (RASA) Research Project initiated in 1996, this report draws in particular on evidence
* Corresponding author. Tel.: þ1 410 516 7500. E-mail address: [email protected] (M.J. Harrower). 0140-1963/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jaridenv.2011.11.020
collected during fieldwork in 2000, 2004, 2005, 2007 and 2008 (McCorriston et al., 2002, 2005, 2011), including radiocarbon assays from alluvium, GPS cross-sections of Wadi Sana, and Digital Elevation Models (DEM) derived from ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) and SRTM (Shuttle Radar Topography Mission) spaceborne imagery. We examine a welldocumented and radiocarbon dated ca. 5200 cal BP shift from aggradation to incision along Wadi Sana and conclude that this change was not solely the result of decreased annual precipitation but was more proximately a function of a concomitant increase in inter and intra-annual precipitation variability (see Dewar and Wallis, 1999; Lézine et al., 2010; Sanlaville, 1992) with attendant shorter duration, high-volume discharges. Moreover, we propose that increasing aridity and resultant plant food scarcity contributed to the emergence of agro-pastoralism in the 6th millennium BP in which small-scale irrigation of drying and incising alluvial sediments supplemented pre-existing herding economies. 2. Context, methods and results A wide variety of environmental factors variable at differing spatio-temporal scales including structural geology, climate, geomorphology and hydrology influenced the range of subsistence opportunities available across ancient southern Arabia. The bedrock of this research area of eastern Yemen’s Hadramawt Governate is an
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uplifted Paleocene-Eocene carbonate platform, exposing at the surface the Umm ar-Radhuma Formation (Beydoun, 1964). This widespread limestone plateau is dissected by Wadi Hadramawt (renamed Wadi Masila east of the city of Tarim) and its tributaries. In northern reaches Umm ar-Rudhuma limestone is overlain by the lower Eocene Jeza Formation limestone; both units are deeply incised, creating a stepped dendritic network with scattered buttes of thin bedded Jeza shale-limestone above the plateau surface and up to three sets of paired terraces incised during episodic postMiocene platform uplift. Wadi Sana, one of the major northflowing tributaries of Wadi Hadramawt/Masila, is formed by a teardrop-shaped watershed at that covers nearly 4000 sq km. South of the town of Ghayl bin Yumain a large sub-basin converges to form the headwaters of Wadi Sana’s main channel (Fig. 1). Throughout the western portion of this sub-basin (hereafter referred to as the Ghayl bin Yumain basin) sizeable expanses of alluvium, gypsum, and marl deposits form a broad flat plain that includes remnants of the Lower-Middle Eocene Rus formation. Presently and in the past the Ghayl bin Yumain basin has been the primary source for alluvial sediments in Wadi Sana. Changes in sediment supply and fluvial transport capacity from this basin, related to variations in regional precipitation and surface-water runoff, were of critical importance in regulating patterns of alluvial deposition and erosion along the middle and lower reaches of Wadi Sana’s main channel (Anderson, 2007). Precipitation across eastern Yemen is primarily a function of southwest monsoon systems that carry moisture from the Indian Ocean during the late spring and summer. These systems are largely controlled by seasonal and longer interval shifts in the position of the Intertropical Convergence Zone (ITCZ), a prominent atmospheric boundary where subtropical and equatorial air masses converge contributing to markedly higher rainfall. Today, limited summer monsoon precipitation (Fisher and Membery, 1998; Sanlaville, 1992) falls on deeply dissected, scree-covered limestone terrain, leading to flashy, short-duration, large volume discharges in valley bottoms that incise and erode alluvial sediments. However, very different
precipitation and runoff conditions must have prevailed during the early Holocene, when precipitation was more frequent and plentiful, and alluvium was aggrading along middle Wadi Sana. For more than 100,000 years these rugged, arid tropical environments were home to human populations who survived as hunters and gatherers (Rose, 2010). In the early Holocene evidence of domesticated animals first appears in the region, including at Wadi Sana’s Manayzah rockshelter (Crassard et al., 2006; Martin et al., 2009; McCorriston and Martin, 2009). Pastoralists who raised sheep, goat and cattle would have relied on moist bottomlands along middle Wadi Sana to sustain their herds, particularly cattle which would have required habitats capable of producing copious grassy forage. A ring of more than 40 cattle skulls discovered by RASA in 2005, excavated and dated to ca. 6300 cal BP reveals the profound economic and social importance of cattle, which nevertheless waned after precipitation declined during the middle Holocene (McCorriston, 2011; McCorriston et al., in press). In concert with declining precipitation near 5200 cal BP, residents of the area began building small irrigation structures to divert water onto arable alluvium (Harrower, 2008a,b). Although we do not know for certain what plants were watered in the small fields they created (Harrower, 2006; McCorriston, 2006), we do know that geomorphology and hydrology of Wadi Sana played an important role in their lives, decisions and choices. 2.1. Archaeological/geological fieldwork Roots of Agriculture in Southern Arabia (RASA) Project fieldwork in winter 2000 and 2004 involved geological and archaeological survey along middle Wadi Sana, focusing in particular on alluvial deposits, as they preserve considerable early to middle Holocene archaeological remains and offer the most promising contexts for preservation of stratified archaeological deposits suitable for excavation. Considerable interest centered on the chronology and geomorphology responsible for alluvial deposition and erosion. Geological inspection and sampling of alluvium, predominantly
Fig. 1. The Wadi Sana watershed of Hadramawt Governate, Yemen. The Ghayl bin Yumain Sub-Basin and locations of 17 GPS cross-sections along middle Wadi Sana indicated (three cross-sections along Wadi Shumlya excluded).
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coarse silt plus fine to medium grain sand (Anderson, 2007; Oches et al., 2001), and both intensive and extensive archaeological survey along Wadi Sana (McCorriston et al., 2005) began to reveal a pattern of early Holocene aggradation followed by a mid-Holocene shift to incision. Of the 39 radiocarbon assays we measured on charcoal buried in alluvium along Wadi Sana, all but two anomalous samples yield ages older than 5144 cal yr BP, indicating that aggradation largely ceased near this time (Harrower, 2006: 106). This pattern is consistent with marine and terrestrial records for the region that show an interval of increased precipitation during the early Holocene, followed by a prominent shift toward aridly near 5500 cal yr BP (Fleitmann et al., 2003, 2007; Lézine et al., 1998, 2007, 2010; Parker et al., 2006; Sirocko et al., 1993; Wilkinson, 1997, 2005). A wide range of natural or cultural factors could potentially be responsible for the shift from aggradation to incision along Wadi Sana, including changes in channel morphology or slope, sediment supply or grain size, water volume, timing or variability of precipitation and runoff, grazing pressure, deforestation, or human manipulation of surface sediment or water resources. 2.2. Topographic and landform analyses Hydro-geospatial analysis can substantially clarify hydrological and geomorphologic conditions relevant to pastoral and agropastoral livelihoods. Most importantly, movements of pastoralists who are often heavily dependent on their herds are closely connected with the availability of water and forage. During the early Holocene water would have been readily available from springs and spring-fed wetland ponds along Wadi Sana, while forage would have required sediments suitable as grass habitat. Similarly later agro-pastoral populations were also dependent on water flow patterns and the spatial distribution of sediments that could be irrigated. Wadi Sana offered relatively attractive opportunities, albeit opportunities that changed considerably through time. Through geomorphic survey and satellite imagery analysis we defined seven landform types: bedrock slopes, scree slopes, plateaus, bedrock terraces, wadi channels, wadi silts, and gravel terraces (Harrower, 2008a; Harrower et al., 2002). The first five of these landforms rarely (if ever) offered suitable habitats for grazing and probably never offered opportunities for cultivation. The final two types, ‘wadi silts’ and ‘gravel terraces’ represent sedimentary deposits that infilled Wadi Sana during the terminal Pleistocene and early Holocene. Wadi silts are a combination of eolian silts and alluvial silty sand that accumulated episodically as atmospheric dust and overbank alluvium deposition. Gravel terrace sediments are pebble, cobble, and boulder gravels deposited in former channels as high-energy bedload sediments. Beginning during the midHolocene incision and erosion left patches of these sedimentary deposits that offer the only suitable grazing and cultivation surfaces available. 2.3. Models of pastoral grazing and agro-pastoral irrigation Two different Digital Elevation Model (DEM) datasets, SRTM and ASTER, were used to analyze terrain conditions relevant to ancient pastoral grazing and agro-pastoral irrigation. First, six separate (one by one degree) scenes of SRTM data were merged to create a DEM consisting of 90 m 90 m grid cells for the entirely of modern day Yemen. Using ArcHydro tools for ArcGIS (Maidment, 2002) a water flow accumulation layer for the same area was then generated by modeling flow from higher to lower elevation DEM grid cells (Harrower, 2009). Among a variety of other insights, this layer reveals that peak flow accumulation at the mouth of Wadi Sana (484,559 grid cells or 3924 sq km) falls above the 99.9th percentile of flow accumulation grid cells in Yemen (the maximum accumulation
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value of 10,271,700 grid cells or 83,201 sq km occurs at the mouth of Wadi Masila as it enters the Indian Ocean near Sayhut). To determine the extent of areas that could have offered suitable grazing territory, the aforementioned SRTM flow accumulation layer was used to produce a drainage network for areas receiving runoff from more than 5000 cells (40.5 sq km). While somewhat subjective, this threshold was selected based on the rule-of-thumb 5000 cell recommendation of Maidment (2002: 73) and first-hand familiarly with the landscape. The resultant dendritic drainage network substantially assists in defining areas of Wadi Sana and its tributaries that could have received sufficient runoff to sustain grasslands. Noticeable, however, are vast differences in the areal extent of bottomlands between bedrock along this network. That is, higher order portions of the network have far wider channel and hence more area available for alluvium to accumulate. To account for this, the network was buffered (i.e. widened) according to Strahler stream order (1st order ¼ 100 m, 2nd ¼ 250 m, 3rd ¼ 400 m, 4th ¼ 550 m) delineating a 7920 ha area of bottomlands along middle and lower Wadi Sana. Overlaying this buffer area on a landform classification layer produced from ASTER satellite imagery (Harrower, 2006, 2008a) shows that wadi silt and gravel terrace landforms (areas that most plausibly could have sustained grass) today comprise approximately 1767 hectares postincision (McCorriston et al., in press), and during the early Holocene (pre-incision) these landforms would have covered a much wider area offering considerable grazing territories for cattle and other wild and domestic ruminants. Alluvial sediments that were initially important for pastoral grazing later become crucial for agro-pastoral irrigation. Previously reported results modeled the distribution of 174 ancient irrigation structures along Wadi Sana identified by archaeological survey (Harrower, 2008a,b, 2010). These analyses employed a higher resolution 15 m DEM extracted from ASTER imagery and show clear associations between locations of irrigation structures, water flow accumulation, and alluvial sediments. Incipient irrigators selected low to moderate flow accumulation areas where water could be slowed and/or diverted onto small plots of wadi silts that served as small 0.5 to 1 hectare fields (Harrower, 2008a). 2.4. Channel cross-section modeling with HEC-RAS To further examine water flow patterns and evaluate potential causes of the middle Holocene shift from aggradation to incision, HEC-RAS software was used to analyze GPS cross-section transects of Wadi Sana (Figs. 2 and 3). During fieldwork in 2004 we measured elevations along twenty linear cross-sections of Wadi Sana using a Trimble Pathfinder ProXRS differential GPS backpack. Given difficulties with noise in the data, in 2008 some cross-sections (T4, T5, T6, T7, T8, T15, T16, T17, T18, T19) were re-measured with a more accurate Trimble 5700 kinematic GPS backpack. Since both GPS receivers were simply measuring a location every second regardless of whether the operator needed to make a detour (e.g., to climb a steep rise or more around a group of acacia trees), all GPS locations were adjusted to reflect elevation along linear cross-sections using the Point Distance function of ArcGIS. These twenty transects, located along a 52.3 km stretch of middle Wadi Sana (Figs. 2 and 3), shed considerable new light on the aggradation/incision transition. Subcritical steady flow modeling was performed using HEC-RAS software (US Army Corps of Engineers, 2010). Boundary condition estimates included a 600 m water surface elevation at the most downstream Wadi Sana cross-section (T26) along with discharge estimates of 700 m3/s and 300 m3/s at the most upstream crosssections of Wadi Sana (T2) and Wadi Shumlya (T4) respectively. Subcritical steady flow analysis run in HEC-RAS then generated water surface elevations along the length of Wadi Sana and Shumlya
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Fig. 2. GPS cross-section T15 shown on a Quickbird satellite image and in a digital photograph as an example of our methods. From foreground to background five different landform types, wadi silts, wadi channel, bedrock slope, plateau and scree slope can be seen in the photograph. After mid-Holocene incision, check dams built in small channels between wadi silt banks and bedrock uplands (including the channel on the extreme right side of the cross-section) were used for irrigation.
as depicted in Fig. 3. Most alluvial sediments were deposited during the early Holocene along middle Wadi Sana where a substantial widening of the area between bedrock uplands and flattening of the channel contributed to reduced transport capacity (i.e. T15, T16, T17, T8, T18, T19). In simple terms, aggradation/incision can be modeled as a balance between sediment supply (or yield) and transport capacity. If input results in a transport capacity that is less than the sediment supply, a channel will tend to aggrade. Alternatively, if input results in a transport capacity that is greater than sediment supply, a channel will tend to erode. As quantified in equation (1), where qs is sediment supply or yield (at time one, qs1, and time two, qs2), q is discharge, S is channel slope, and D is sediment caliber (Clark and Wilcock, 2000), sediment caliber increases promote aggradation, while discharge and slope increases promote incision.
qs1 ¼ qs2
q1 q2
2 2 3=2 S1 D2 S2 D1
(1)
These considerations, along with HEC-RAS analyses, allow us to rule out a number of potential causes for the mid-Holocene shift to incision. Along its length Wadi Sana exhibits no major slope
changes, running a roughly 3.4% grade from an elevation of 774 m to 597 m over 52.3 km (Fig. 4). Similarly, sediment caliber is unlikely to have changed significantly, as the Ghayl bin Yumain basin remained the primary source of alluvial sediment throughout the Holocene (Anderson, 2007). Therefore S and D parts of the equation factor to 1, and manipulating q heuristically shows that a reduction in discharge (as a result of decreasing mean annual precipitation) would lead to reduced transport capacity and thus likely aggradation rather than degradation. Concordantly, one would expect that the mid-Holocene decrease in annual precipitation identified in numerous independent paleoclimatic records (e.g. Fleitmann et al., 2007; Lézine et al., 2010; Parker et al., 2006) would have promoted aggradation rather than incision. Notably, a drop to ½ the original early Holocene discharge would be capable of transporting only about ¼ of the original amount of sediment (see Wilson, 1973). A decrease in precipitation alone is therefore unlikely to have caused the profound shift from aggradation to incision we documented during fieldwork. The most plausible alternative (that is left unaccounted for if one considers Eq. (1) only in terms of unidirectional increases or decreases in precipitation) is oscillating inter and intra-annual precipitation changes. It has long been known that annual precipitation variability typically increases
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Fig. 3. GPS cross-sections of Wadi Sana from upstream to downstream. All cross-sections plotted on a common scale using HEC-RAS software (note: scale of X and Y axes differ, three cross-sections of Wadi Shumlya excluded). The shaded area represents estimated subcritical flow using a 600 m water surface elevation for the most downstream crosssection (T26) and discharge estimates of 700 m3/s and 300 m3/s for the most upstream cross-sections of Wadi Sana (T2) and Wadi Shumlya (T4) respectively. A substantial widening of the area between bedrock uplands along middle Wadi Sana evident from T15, T16, T17, T8, T18 to T19 led to reduced transport capacity and resultant early Holocene alluvial deposition in that area.
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Fig. 3. (continued).
Wad_Sana Reach2
Wadi_Sana Reach1
W a d i _ S h u m l y a
800
750
R e a c h 1
Legend Ground
Elevation (m)
700
650
600
550 0
10000
20000
30000
40000
50000
60000
Main Channel Distance (m)
Fig. 4. Middle Wadi Sana profile plot generated by HEC-RAS software. Plot shows channel slope along Wadi Sana and Wadi Shumlya as measured by 20 GPS cross-sections.
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as mean precipitation declines, particularly in the tropics (Dewar and Wallis, 1999). Thus we propose that it was not solely a decline in precipitation, but a concomitant increase in precipitation variability that is most proximately responsible for the shift from aggradation to incision along middle Wadi Sana. Indeed, if a change in rainfall regimes was responsible for incision, then either precipitation events were shorter and more intense or regional models are not correctly identifying localized increases in mean annual precipitation for Wadi Sana; given the comparative wealth of data for a mid-Holocene shift to aridity we consider the former far more likely. 3. Conclusions: hydro-geospatial changes and pastoral/agro-pastoral landscapes The spatio-temporal distribution of water flow across ancient south Arabian landscapes was of central importance to ancient herders and transhumant farmers. New geospatial datasets and technologies offer a wealth of analytical opportunities. We have used GIS modeling tools to shed new light on the mid-Holocene shift from aggradation to incision along ancient Wadi Sana. Though in some previous publications we interpreted this shift simply as the result of decreased precipitation, present analyses suggest that it was not only a decrease in annual water volume, but a likely increase in the seasonal and annual variability of precipitation, including episodic high discharge events that would have been most important in instigating the shift to erosion and incision. Intervening low flow conditions may have been unable to mobilize and transport even fine-grained sediment from the Ghayl bin Yumain basin (Anderson, 2007) and therefore do not appear to have offset the erosive effect of periodic high volume flows. The long-term dynamics of water and sediment along Wadi Sana held considerable importance for ancient residents of the area. Wet bottomlands would have been crucial to families with cattle, and the drying and erosion of those areas would have contributed to changing lifeways, including the eventual disappearance of cattle (in favor of sheep, goats and eventually camels), along with experimentation and reliance on small-scale irrigation. Alluvial sediments were similarly important to agro-pastoralists, and as middle Wadi Sana began incising and wadi silt sediments became stranded high ground irrigation strategies were devised to ensure that remnant sediments surfaces were sufficiently watered. Though we certainly would not suggest that environmental changes determined trajectories of human history, they undoubtedly did play a substantive role in shaping the range and expression of different options and geospatial technologies hold a promising future in explicating ancient choices and human-environment relationships. Acknowledgements This paper greatly benefited from the combined efforts of numerous students and scholars who contributed to the Roots of Agriculture in Southern Arabia (RASA) research project, most notably Joshua Anderson whose thesis laid the groundwork for these analyses. We are particularly grateful to the Yemeni General Organization for Antiquities and Museums including Abdullah Ba Wazir, Abdalaziz Bin ’Aqil, Khalid Ba Dhofary, Ietha Al-Amary, and Abdalbaset Noman. We also thank Canadian Nexen Petroleum, particularly Gregor Mawhinney, Alan Brindley, Kevin Tracy, Rick Jensen, Dave Smith, and Kevin Marlow for enormous logistical support over nearly a decade. Thanks also to Peter Wilcock who offered comments and advice that proved central to the analyses of this paper.
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