Peter J. Fawcett

Terrestrial water fluxes dominated by transpiration

Renewable fresh water over continents has input from precipitation and losses to the atmosphere through evaporation and transpiration. Global-scale estimates of transpiration from climate models are poorly constrained owing to large uncertainties in stomatal conductance and the lack of catchment-scale measurements required for model calibration, resulting in a range of predictions spanning 20 to 65 per cent of total terrestrial evapotranspiration (14,000 to 41,000 km3 per year) (refs 1, 2, 3, 4, 5). Here we use the distinct isotope effects of transpiration and evaporation to show that transpiration is by far the largest water flux from Earth’s continents, representing 80 to 90 per cent of terrestrial evapotranspiration. On the basis of our analysis of a global data set of large lakes and rivers, we conclude that transpiration recycles 62,000 ± 8,000 km3 of water per year to the atmosphere, using half of all solar energy absorbed by land surfaces in the process. We also calculate CO2 uptake by terrestrial vegetation by connecting transpiration losses to carbon assimilation using water-use efficiency ratios of plants, and show the global gross primary productivity to be 129 ± 32 gigatonnes of carbon per year, which agrees, within the uncertainty, with previous estimates. The dominance of transpiration water fluxes in continental evapotranspiration suggests that, from the point of view of water resource forecasting, climate model development should prioritize improvements in simulations of biological fluxes rather than physical (evaporation) fluxes.

Extended megadroughts in the southwestern United States during Pleistocene interglacials

The potential for increased drought frequency and severity linked to anthropogenic climate change in the semi-arid regions of the southwestern United States (US) is a serious concern. Multi-year droughts during the instrumental period and decadal-length droughts of the past two millennia were shorter and climatically different from the future permanent, ‘dust-bowl-like’ megadrought conditions, lasting decades to a century, that are predicted as a consequence of warming. So far, it has been unclear whether or not such megadroughts occurred in the southwestern US, and, if so, with what regularity and intensity. Here we show that periods of aridity lasting centuries to millennia occurred in the southwestern US during mid-Pleistocene interglacials. Using molecular palaeotemperature proxies to reconstruct the mean annual temperature (MAT) in mid-Pleistocene lacustrine sediment from the Valles Caldera, New Mexico, we found that the driest conditions occurred during the warmest phases of interglacials, when the MAT was comparable to or higher than the modern MAT. A collapse of drought-tolerant C4 plant communities during these warm, dry intervals indicates a significant reduction in summer precipitation, possibly in response to a poleward migration of the subtropical dry zone. Three MAT cycles ∼2 °C in amplitude occurred within Marine Isotope Stage (MIS) 11 and seem to correspond to the muted precessional cycles within this interglacial. In comparison with MIS 11, MIS 13 experienced higher precessional-cycle amplitudes, larger variations in MAT (4–6 °C) and a longer period of extended warmth, suggesting that local insolation variations were important to interglacial climatic variability in the southwestern US. Comparison of the early MIS 11 climate record with the Holocene record shows many similarities and implies that, in the absence of anthropogenic forcing, the region should be entering a cooler and wetter phase.

The pronounced seasonality of global groundwater recharge

Groundwater recharged by meteoric water supports human life by providing two billion people with drinking water and by supplying 40% of cropland irrigation. While annual groundwater recharge rates are reported in many studies, fewer studies have explicitly quantified intra-annual (i.e., seasonal) differences in groundwater recharge. Understanding seasonal differences in the fraction of precipitation that recharges aquifers is important for predicting annual recharge groundwater rates under changing seasonal precipitation and evapotranspiration regimes in a warming climate, for accurately interpreting isotopic proxies in paleoclimate records, and for understanding linkages between ecosystem productivity and groundwater recharge. Here we determine seasonal differences in the groundwater recharge ratio, defined here as the ratio of groundwater recharge to precipitation, at 54 globally distributed locations on the basis of 18O/16O and 2H/1H ratios in precipitation and groundwater. Our analysis shows that arid and temperate climates have wintertime groundwater recharge ratios that are consistently higher than summertime groundwater recharge ratios, while tropical groundwater recharge ratios are at a maximum during the wet season. The isotope-based recharge ratio seasonality is consistent with monthly outputs from a global hydrological model (PCR-GLOBWB) for most, but not all locations. The pronounced seasonality in groundwater recharge ratios shown in this study signifies that, from the point of view of predicting future groundwater recharge rates, a unit change in winter (temperate and arid regions) or wet season (tropics) precipitation will result in a greater change to the annual groundwater recharge rate than the same unit change to summer or dry season precipitation.

Large Holocene lakes and climate change in the Chihuahuan Desert

Lake-level variations preserved as beach ridges in the Laguna El Fresnal and Laguna Santa Maria subbasins, northern Mexico, record millennially spaced episodes of increased precipitation during the Holocene epoch. We find that the early, middle, and late Holocene were punctuated by periods wet enough to establish large pluvial lakes in currently dry basins in the Chihuahuan Desert; the largest dated pluvial lake covered ∼5650 km2 during the early Holocene. Constructional beach ridges in these subbasins are 221 ± 33 14C yr B.P. (Little Ice Age equivalent), 3815 ± 52 to 4251 ± 59 14C yr B.P. (early Neoglacial), 6110 ± 80 to 6721 ± 68 14C yr B.P. (mid-Holocene), and 8269 ± 64 to 8456 ± 97 14C yr B.P. (early Holocene), dates that correlate with other millennially spaced wet or cold events in the Northern Hemisphere. We attribute these wet episodes to increased precipitation, cooler temperatures, and reduced evaporation following southward shifts in winter storm tracks, which are related to long-term El Nino–Southern Oscillation variability during the Holocene.

15 k.y. paleoclimatic and glacial record from northern New Mexico

The southern Sangre de Cristo Mountains, New Mexico, contain evidence of glacial activity from the late Pleistocene to late Holocene. Sediment cores recovered from an alpine bog (3100 m) trapped behind a Pinedale age moraine, ∼2 km downvalley from a high-elevation cirque, reached glacial-age debris and recovered ∼6 m of lake clays overlain by gyttja. Accelerator mass spectrometry dating, sedimentology, variations in magnetic properties, and organic carbon data reveal six distinct periods of glacial and/or periglacial activity. These include a late Pleistocene Pinedale glacial termination just before 12120 1 4 C yr B.P., a Younger Dryas chron cirque glaciation, an early Neoglacial periglacial event (ca. 4900 1 4 C yr B.P.), a late Holocene cirque glaciation (3700 1 4 C yr B.P.), as well as late Holocene periglacial events at 2800 1 4 C yr B.P. and the Little Ice Age (ca. 120 1 4 C yr B.P.). Cold events in the middle to late Holocene correlate with subtle ice-rafting events in the North Atlantic and records of cold events in North America and Europe and were probably hemispheric in extent.

Chronotopographic analysis directly from point-cloud data: A method for detecting small, seasonal hillslope change, Black Mesa Escarpment, NE Arizona

The advent of high-resolution, precise, back-pack portable terrestrial lidar scanners (TLS) provides a revolutionary new tool for obtaining quantitative, high-resolution (2-mm to 30-mm point spacing) measurements of landscape surface features. Moreover, data collected using these instruments allow observation of geomorphic processes in systems that can experience change on a daily basis. We have introduced TLS techniques in ongoing investigations of semiarid landscapes associated with weakly cemented sandstones along part of the Black Mesa escarpment of NE Arizona. Clay-cemented, Jurassic sandstones exposed along this escarpment are sensitive to moisture, and thus climate, via hydration-expansion weathering of interstitial clay. Sediment shed from weathered slopes has caused locally rapid valley floor aggradation and upper basin slope vertical denudation rates of 2–3 mm/yr over 10- to 100-yr timescales, as indicated by dendrochronology coupled with soil geomorphic analysis. These rates suggest rapid hillslope denudation rates. Employing the University of New Mexico Lidar Laboratory Optech Ilris 3D TLS, we are constructing a high-resolution model of two major basins along the escarpment. Focusing on a single, small (30 × 60 m) area of a mostly non-vegetated, steep slope (>35°), we demonstrate in this paper a method of comparative analysis of point-cloud data sets that can detect subcentimeter change resulting from a single season of monsoon precipitation along the escarpment. Using repeat scans can provide an empirical evaluation of single season erosion rates in the study site, and because our observations are geospatial in nature, we can also document the parts of the slopes that make the greatest contribution to local valley floor aggradation. In demonstrating the utility of this method, we expect that continued investigation of this site will provide insight to the key processes associated with soil-mantled versus bedrock-dominated slopes during modern escarpment retreat and hillslope modification, which, in turn, may further elucidate the impacts of Holocene climate change on this rapidly evolving landscape.

Jasechko et al. reply

replying to A. M. J. Coenders-Gerrits et al. 506, http://dx.doi.org/10.1038/nature12925 (2014)In their Comment, Coenders-Gerrits et al. suggest that our conclusion that transpiration dominates the terrestrial water cycle is biased by unrepresentative input data and optimistic uncertainty ranges related to runoff, interception and the isotopic compositions of transpired and evaporated moisture. We clearly presented the uncertainties applied in our Monte-Carlo sensitivity analysis, we reported percentile ranges of results rather than standard deviations to best communicate the nonlinear nature of the isotopic evaporation model, and we highlighted that the uncertainty in our calculation remains large, particularly in humid catchments (for example, figure 2 in our paper).