No abstract

Drought has been a recurrent phenomenon in Mexico. For its assessment and monitoring, several studies have monitored meteorological droughts using standardized indices of precipitation deficits. Such conventional studies have mostly relied on rain gauge-based measurements, with the main limitation being the scarcity of rain gauge spatial coverage. This issue does not allow a robust spatial characterization of drought. A recent alternative for monitoring purposes can be found in satellite-based remote sensing of meteorological variables. The main objective of this study is to evaluate the standardized precipitation index (SPI) in Mexico during the period 1998 to 2013, using the Tropical Rainfall Measuring Mission (TRMM) satellite product 3B42. Results suggest that Mexico experienced the driest conditions during the great drought between 2011 and 2012; however, temporal variability in the SPI was found across different climatic regions. Nevertheless, a comparison of the SPI derived by TRMM against the rain gauge-based SPI computed by the official Mexican Drought Monitor showed low to medium correlation of the time series though both SPIs managed to capture the most relevant droughts at the national scale. We conclude that the TRMM product can properly monitor meteorological droughts despite its relative short dataset length (~15 years). Finally, we recommend an assimilation of rain gauge and satellite-based precipitation data to provide more robust estimates of meteorological drought severity.

The impacts of climate extremes and water use on groundwater storage across large aquifers can be quantified using Gravity Recovery and Climate Experiment (GRACE) satellite monitoring. We present new methods to improve estimates of changes in groundwater storage by incorporating irrigation soil moisture corrections to common data assimilation products. These methods are demonstrated using data from the High Plains Aquifer (HPA) for 2003 to 2013. Accounting for the impacts of observed and inferred irrigation on soil moisture significantly improves estimates of groundwater storage changes as verified by interpolated measurements from~10,000 HPA wells. The resulting estimates show persistent declines in groundwater storage across the HPA, more severe in the southern and central HPA than in the north. Groundwater levels declined by an average of approximately 276 ± 23 mm from 2003 to 2013, resulting in a storage loss of 125 ± 4.3 km 3 , based on the most accurate of the three methods developed here.

Understanding how ecosystem functioning affects hydrological partitioning at the catchment scale is critically important to predict the annual water balance under climate-related land use change. Terrestrial ecosystems rely on rainfall infiltration while riparian ecosystems rely on the accumulation of surface and subsurface runoff in the riparian zone and the channel network. Some of the rainfall that infiltrates into the soils will be available for plants on the catchment's hillslopes. Some of that water may percolate down the soil profile where it can recharge a perched shallow aquifer or move even deeper to recharge bedrock aquifers. Lateral shallow subsurface flow provides a subsidy of water to plants downslope, which allows different plant species to occupy the toeslopes (Thompson et al., 2011a; Hwang et al., 2012). Perched and deep aquifers can sustain streamflow during dry periods and are thus essential for riparian and aquatic ecosystems. At the catchment scale, the persistence of flow can be analyzed by means of the flow duration curve (a plot that shows the percentage of time that discharge in a stream is likely to equal or exceed some specified value of interest).Climate has a first order control on the annual water balance (Budyko, 1974). The aridity index (ratio of average annual potential evapotranspiration, PET, to average annual rainfall, P) can predict the evaporative fraction and the runoff coefficient at climate time scales (~30 years) in many catchments. Less is known about how catchment ecosystems affect the annual water balance. Huxman et al. (2004) showed that when biomes undergo drought conditions their rainwater use efficiency (Net Primary Production/P) converges to a common maximum value. In a similar context, Troch et al. (2009) introduced the Horton index (the ratio of vaporization (i.e. evapotranspiration) to catchment wetting (the amount of water that infiltrates into the soils and does not runoff superficially) as a measure of rainwater use efficiency of the catchment's ecosystems, and showed that the Horton Index (HI) converges to a value of 1 when catchments undergo drought conditions. Subsequent research about the HI found that the index is controlled by climate (aridity index) and landscape characteristics (catchment slope and elevation) and is a useful first-order predictor for annual and interannual vegetation greenness at the catchment scale (Brooks et al., 2011; Voepel et al., 2011). 3The role of catchment water storage on vegetation response has been acknowledged in several recent ecohydrological studies (Tague, 2009; Miller et al., 2010; Thompson et al., 2011a). Evidence that water storage subsidizes plant water use through lateral hydrological connectivity (driven by topography) has been shown at the plot and the catchment scale and across different biomes (Scott et al., 2008; Thompson et al., 2011b; Hwang et al., 2012). Past research carried out in a temperate climate experimental watershed suggests a direct relationship between landscape position and storage that e...