on the web D rought is a period of deficient precipitation with impacts on agriculture, water resources, and the natural ecosystems. It is the natural hazard that affects more people with the most negative consequences in the world, being responsible for extreme economic loss, famine, epidemics, and land degradation. In many developing countries, drought increases structural problems, causing unemployment, impoverishment, decreases in crop yields, and even forced migrations. Thus, improving our knowledge about the spatial and temporal variability of drought is a fundamental prerequisite to quantify the drought hazard and vulnerability of different systems and regions, with the final purpose of improving drought mitigation and preparedness. The availability of data for characterizing drought conditions over given regions and time periods is therefore in high demand by scientists and managers at different levels. Such datasets must be operative both for analyzing past droughts (their onset and end, magnitude, and spatial extent) and for determining drought probabilities and vulnerabilities across a wide range of systems.A critical issue in the study of drought vulnerability is the multiscalar nature of drought, since the response of the hydrological (soil moisture, groundwater, river discharge, reservoir storage, etc.) and biological (crops, natural vegetation, etc) systems to water shortage varies markedly and with different response times. This explains why severe drought conditions can be recorded in one system (e.g., low river flows), whereas other systems in the same region (e.g., crops) have normal or even humid conditions. Thus, the time scale over which the water deficit accumulates becomes extremely important, and functionally separates hydrological, environmental, agricultural, and other types of drought.Recently, a new drought indicator, the Standardised Precipitation-Evapotranspiration Index (SPEI), developed by Vicente-Serrano et al. in a 2010 Journal of Climate article, has been proposed to quantify the drought condition over a given area. The SPEI considers not only precipitation but also evapotranspiration (PET) data in its calculation, allowing for a more complete approach to explore the effects of climate change on drought conditions. The SPEI can be calculated at several time scales to adapt to the characteristic times of response to drought of target natural and economic systems, determining their resistance to drought.Following the development of SPEI, a new global dataset, the SPEIbase, has been made available to the scientific community. The dataset covers the period 1901-2006 with a monthly frequency, and offers global coverage at a 0.5-degree resolution. The dataset consists of the monthly values of the SPEI at time scales from 1 to 48 months. This article describes the SPEIbase and shows some of its potential uses.A complete description of the data and metadata, and links to download the files, are provided at http:// sac.csic.es/spei. The STandardiSed PreciPiTaTionevaPoTranSPiraTion inde...

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Abstract. Optical disdrometers are present weather sensors with the ability of providing detailed information on precipitation such as rain intensity, radar reflectivity or kinetic energy, together with discrete information on the particle size and fall velocity distribution (PSVD) of the hydrometeors. Disdrometers constitute a step forward towards a more complete characterization of precipitation, being useful in several research fields and applications. In this article the performance of two extensively used optical disdrometers, the most recent version of OTT Parsivel 2 disdrometer and Thies Clima Laser Precipitation Monitor (LPM), is evaluated. During 2 years, four collocated optical disdrometers, two Thies Clima LPM and two OTT Parsivel 2 , collected up to 100 000 min of data and up to 30 000 min with rain in more than 200 rainfall events, with intensities peaking at 277 mm h −1 in 1 minute. The analysis of these records shows significant differences between both disdrometer types for all integrated precipitation parameters, which can be explained by differences in the raw PSVD estimated by the two sensors. Thies LPM recorded a larger number of particles than Parsivel 2 and a higher proportion of small particles than OTT Parsivel 2 , resulting in higher rain rates and totals and differences in radar reflectivity and kinetic energy. These differences increased greatly with rainfall intensity. Possible causes of these differences, and their practical consequences, are discussed in order to help researchers and users in the choice of sensor, and at the same time pointing out limitations to be addressed in future studies.

Most applications of the extreme value (EV) theory have assumed stationarity, i.e. the statistical properties of the process do not change over time. However, there is evidence suggesting that the occurrence of extreme events is not stationary but changes naturally, as it has been found for many other climate variables. Of paramount importance for hazard analysis is whether the observed precipitation time series exhibit long-term trends or cycles; such information is also relevant in climate change studies. In this study, the theory of non-stationary extreme value (NSEV) analysis was applied to data series of daily precipitation using the peaks-over-threshold (POT) approach. A Poisson/generalized Pareto (P/GP) model, in which the model parameters were allowed to vary linearly with time, was fitted to the resulting series of precipitation event's intensity and magnitude. A log-likelihood ratio test was applied to determine the existence of trends in the model parameters. The method was applied to a case study in northeast Spain, comprising a set of 64 daily rainfall series from 1930 to 2006. Statistical significance was achieved in less than 5% of the stations using a linear non-stationary model at the annual scale, indicating that there is no evidence of a generalized trend in extreme precipitation in the study area. At the seasonal scale, however, a significant number of stations along the Mediterranean (Catalonia region) showed a significant decrease of extreme rainfall intensity in winter, while experiencing an increase in spring.

Soil erosion is a serious ecological and environmental problem, and the main cause of land degradation in many ecosystems at global scale. Detachment of soil particles by raindrop splash is the first stage in the soil erosion process. A review of the scientific literature published in peer-reviewed international journals (ISI) over the last decades on splash erosion research sheds light on the current scientific knowledge on this topic. In addition, it highlights the research gaps and unanswered questions in our understanding of soil erosion processes due to splash. In this literature review, a bibliographic search in Web of Science by the Institute for Scientific Information (ISI) database was carried out on August the 9th, 2016, that returned 669 papers containing the words "splash erosion". The research found was categorised according to a number of criteria: i) devices used to measure splash erosion, ii) advantages and disadvantages of these devices, iii) splash erosion studies by country, iv) date of publication of the first article, v) evolution of the number of articles published in each ten-year period, vi) concepts studied, vii) keywords, viii) authors, ix) number of citations, and x) most cited articles. After this review a synthesis of the information that the science has published about splash erosion was made in order to improve our understanding about splash erosion, by identifying the research questions that still remain unanswered today about the first detachment mechanism. From this review several issues were found important for the advancement of this research topic: a) further study of the known basic factors influencing splash erosion; b) description and quantification of sources of uncertainty about the measurement of different variables; c) to understand the influences that the chosen research approach by individual researchers will have in the final result; and, d) to study the impact of drivers or mitigation techniques that may affect splash erosion.