Droughts and aridity in northeast Brazil

ABSTRACT. Results of a study of droughts and aridity in northeast Brazil are presented in this paper. The study is based on the analysis of yearly water balances at fifteen stations in the region. Incidence of droughts of varying intensities at the stations is discussed. Climatic shifts at the stations are evaluated on the basis of the moisture index values. The use of the cumulative deviation technique in drought studies is briefly mentioned.

Fast Pliocene integration of the Central Anatolian Plateau drainage: Evidence, processes, and driving forces

Continental sedimentation was widespread across the Central Anatolian Plateau in Miocene–Pliocene time, during the early stages of plateau uplift. Today, however, most sediment produced on the plateau is dispersed by a well-integrated drainage and released into surrounding marine depocenters. Residual long-term (106–107 yr) sediment storage on the plateau is now restricted to a few closed catchments. Lacustrine sedimentation was widespread in the Miocene–Pliocene depocenters. Today, it is also restricted to the residual closed catchments. The present-day association of closed catchments, long-term sediment storage, and lacustrine sedimentation suggests that the Miocene–Pliocene sedimentation also occurred in closed catchments. The termination of sedimentation across the plateau would therefore mark the opening of these closed catchments, their integration, and the formation of the present-day drainage. By combining newly dated volcanic markers with previously dated sedimentary sequences, we show that this drainage integration occurred remarkably rapidly, within 1.5 m.y., at the turn of the Pliocene. The evolution of stream incision documented by these markers and newly obtained 10Be erosion rates allow us to discriminate the respective con­tributions of three potential processes to drainage integration, namely, the capture of closed catch­ments by rivers draining the outer slopes of the plateau, the overflow of closed lakes, and the avul­sion of closed catchments. Along the southern plateau margin, rivers draining the southern slope of the Central Anatolian Plateau expanded into the plateau interior; however, only a small amount of drainage integration was achieved by this process. Instead, avulsion and/or overflow between closed catchments achieved most of the integration, and these top-down processes left a distinctive sedi­mentary signal in the form of terminal lacustrine limestone sequences. In the absence of substantial regional climate wetting during the early Pliocene, we propose that two major tectonic events triggered drainage inte­gration, separately or in tandem: the uplift of the Central Anatolian Plateau and the tectonic com­pletion of the Anatolian microplate. Higher surface uplift of the eastern Central Anatolian Plateau relative to the western Central Anatolian Plateau promoted more positive water balances in the eastern catchments, higher water discharge, and larger sediment fluxes. Overflow/avulsion in some of the eastern catchments triggered a chain of avulsions and/or overflows, sparking sweeping integration across the plateau. Around 5 Ma, the inception of the full escape of the Anatolian microplate led to the disruption of the plateau surface by normal and strike-slip faults. Fault scarps partitioned large catchments fed by widely averaged sediment and water influxes into smaller catchments with more contrasted water balances and sediment fluxes. The evolution of the Central Anatolian Plateau shows that top-down processes of integration can outcompete erosion of outer plateau slopes to reintegrate plateau interior drainages, and this is overlooked in current models, in which drainage evolution is dominated by bottom-up integration. Top-down integration has the advantage that it can be driven by more subtle changes in climatic and tectonic boundary conditions than bottom-up integration.

Location, location, location – Considering local neighborhood when analyzing large samples of UK catchments

<p>Studying large samples of catchments has been an effective means for comparative hydrology as it provides a wide range of hydrological conditions which can be used to learn similarities and differences between places. Such analyses typically include an attempt to organize catchments along some gradient (e.g. climate) or in clusters (e.g. geology) using catchment descriptors (e.g. an aridity index). Various past studies have pointed to the problem that available catchment descriptors are often not sufficient to capture hydrologically relevant catchment behaviours. It is further widely acknowledged that the water balance of many catchments is not closed. Several hypotheses for the causes of this lack of closed water balance are stated in literature.</p><p>If we assume that the dominant control on water balance is climate, then catchments’ water balances should change smoothly in space (since the climate varies smoothly). If they do not, then something else must be controlling this behaviour. We expect that size, location and geology might play important role in the water balances of UK catchments. We aim to study the differences in water balance between catchments to understand the role of catchment location. We test different hypotheses while considering the local neighborhood of 669 UK catchments from the CAMELS-GB dataset.</p>

Reducing Solar Radiation Forcing Uncertainty and Its Impact on Surface Energy and Water Fluxes

Downward shortwave radiation ( Rsd ) determines the surface energy balance, alters evapotranspiration and hydrological conditions, and feeds back to the regional and global climate. Large-scale Rsd estimates are usually retrieved from satellite based on the top-of-atmosphere radiation and cloud parameters. These estimates are subject to biases and temporal inhomogeneity due to errors in atmospheric parameters, algorithms, and sensor changes. We found that three satellite products overestimate Rsd by 8-10% over Asia for 1984-2006, particularly in high latitudes. We used the Model Ensemble Tree (MTE) machine-learning algorithm and commonly-used ensemble averaging methods to integrate ground observations and satellite products. Validations based on test stations and independent networks showed that the MTE approach reduces the median relative biases from 8-10% to 2%, which is more effective than the ensemble averaging methods. We further evaluated the impacts of uncertainty in radiation forcing on surface energy and water balances using the land surface model Noah-MP. The uncertainty of radiation data affects the prediction of sensible heat the most, and also largely affects latent heat prediction in humid regions. Holding the other variables constant, a 10% positive bias in Rsd can lead to a 20% to 60% positive bias in the monthly median sensible heat. The simulated hydrological responses to changing radiation forcing are nonlinear as a result of the interactions among ET, snowpack, and soil moisture. Our analysis concludes that reducing uncertainty of radiation data is beneficial for predicting regional energy and water balances, which requires more high-quality ground observations and improved satellite retrieval algorithms.