James McPhee

Changes of glaciers in the Andes of Chile and priorities for future work

Glaciers in the Andes of Chile seem to be shrinking and possibly loosing mass, but the number and types of studies conducted, constrained mainly by data availability, are not sufficient to provide a synopsis of glacier changes for the past or future or explain in an explicit way causes of the observed changes. In this paper, we provide a systematic review of changes in glaciers for the entire country, followed by a discussion of the studies that have provided evidence of such changes. We identify a missing type of work in distributed, physically-oriented modelling studies that are needed to bridge the gap between the numerous remote sensing studies and the specific, point scale works focused on process understanding. We use an advanced mass balance model applied to one of the best monitored glaciers in the region to investigate four main research issues that should be addressed in modelling studies for a sound assessment of glacier changes: 1) the use of physically-based models of glacier ablation (energy balance models) versus more empirical models (enhanced temperature index approaches); 2) the importance of the correct extrapolation of air temperature forcing on glaciers and in high elevation areas and the large uncertainty in model outputs associated with it; 3) the role played by snow gravitational redistribution; and 4) the uncertainty associated with future climate scenarios. We quantify differences in model outputs associated with each of these choices, and conclude with suggestions for future work directions.

Validation and Error Characterization of the GPCP-1DD Precipitation Product over the Contiguous United States

Abstract A validation and error characterization study of the Global Precipitation Climatology Project, 1 degree daily (GPCP-1DD) precipitation product over the contiguous United States is presented. Daily precipitation estimates over a 1° grid are compared against aggregated precipitation values obtained from the forcing field of the North American Land Data Assimilation System (LDAS). LDAS daily values are consistent with the National Centers for Environmental Prediction Climate Prediction Center (CPC) gauge-based daily precipitation product and hence are regarded as realistic ground-truth values with full coverage of the United States. Continuous and categorical measures of skill are presented, so that both the ability of GPCP-1DD to identify a precipitation event and its accuracy in determining cumulative precipitation amounts are evaluated. Daily values are aggregated into seasonal averages, and spatial averages are computed for five arbitrarily defined zones that cover most of the study area. Result...

Physically Based Mountain Hydrological Modeling Using Reanalysis Data in Patagonia

A physically based hydrological model for the upper Baker River basin (UBRB) in Patagonia was developed using the modular Cold Regions Hydrological Model (CRHM) in order to better understand the processes that drive the hydrological response of one of the largest rivers in this region. The model includes a full suite of blowing snow, intercepted snow, and energy balance snowmelt modules that can be used to describe the hydrology of this cold region. Within this watershed, snowfall, wind speed, and radiation are not measured; there are no high-elevation weatherstations;and existingweatherstations aresparselydistributed. Theimpactofatmospheric datafromECMWFinterimreanalysis(ERA-Interim) andClimate Forecast System Reanalysis (CFSR) on improving model performance by enhancing the representation of forcing variables was evaluated. CRHM parameters were assigned for local physiographic and vegetation characteristics based on satellite land cover classification, a digital elevation model, and parameter transfer from cold region environments in western Canada. It was found that observed precipitation has almost no predictive power [Nash‐ Sutcliffecoefficient(NS),0.3]whenusedtoforcethehydrologicmodel,whereasmodelperformanceusingany of the reanalysis products—after bias correction—was acceptable with very little calibration (NS . 0.7). The modeled water balance shows that snowfall amounts to about 28% of the total precipitation and that 26% of totalriverflowstemsfromsnowmelt.Evapotranspirationlossesaccountfor7.2%oftotalprecipitation,whereas sublimationandcanopyinterceptionlossesrepresentabout1%.Thesoilcomponentisthedominantmodulator of runoff, with infiltration contributing as much as 73.7% to total basin outflow.

Altitudinal gradients, midwinter melt, and wind effects on snow accumulation in semiarid midlatitude Andes under La Niña conditions

The Andes Cordillera remains a sparsely monitored and studied snow hydrology environment in comparison to similar mountain ranges in the Northern Hemisphere. In order to uncover some of the key processes driving snow water equivalent (SWE) spatial variability, we present and analyze a distributed SWE data set, sampled at the end of accumulation season 2011. Three representative catchments across the region were monitored, obtaining measurements in an elevation range spanning 2000 to 3900 m asl and from 32.4° to 34.0°S in latitude. Climatic conditions during this season corresponded to a moderate La Nina phenomenon, which is generally correlated with lower-than normal accumulation. Collected measurements can be described at the regional and watershed extents by altitudinal gradients that imply an increase by a factor of two in snow depth between 2200 and 3000 m asl, though with significant variability at the upper sites. In these upper sites, we found north-facing, wind-sheltered slopes showing 25% less average SWE values than south-facing, wind-exposed ones. This suggests that under these conditions, solar radiation dominated wind transport effects in controlling end-of-winter variability. Nevertheless, we found clusters of snow depth measurements above 3000 m asl that can be explained by wind exposure differences. This is the first documented snow depth data set of this spatial extent for this region, and it is framed within an ongoing research effort aimed at improving understanding and modeling of snow hydrology in the extratropical Andes Cordillera.

Catastrophic, rainfall-induced debris flows in Andean villages of Tarapacá, Atacama Desert, northern Chile

In March 2012, during the rainy season in the Altiplano plateau, a >100-year return period rainfall event affected the deeply incised valleys of the Precordillera of the Tarapacá Region, northern Chile. This extreme event in a very arid region triggered a number of debris and mud flows that caused severe damage and destruction in several small villages along the Camiña and Tarapacá valleys. The highly vulnerable location of the villages on top of alluvial fans due to socioeconomic and cultural reasons is a key factor to explain the level of destruction in most villages. In this paper, this unusual, remarkable landslide event is described, and the hazard faced by these settlements for future rainfall episodes and possible mitigation measures are discussed.

Spatiotemporal Variations in Hydroclimate across the Mediterranean Andes (30°–37°S) since the Early Twentieth Century

AbstractIn the Mediterranean Andes region (MA; 30°–37°S), the main rivers are largely fed by melting snowpack and provide freshwater to around 10 million people on both sides of the Andes Mountains. Water resources in the MA are under pressure because of the extensive development of industrial agriculture and mining activities. This pressure is increasing as the region faces one of its worst recorded droughts. Previous studies have pointed to El Nino–Southern Oscillation (ENSO) as the main climatic force impacting the MA. However, the role of decadal and multidecadal climate variability, their spatial patterns, and the recurrence of long-term droughts remains poorly studied. In an attempt to better understand these factors, spatial and temporal patterns of hydroclimatic variability are analyzed using an extensive database of streamflow, precipitation, and snowpack covering the period between 1910 and 2011. These analyses are based on the combination of correlation, principal components, and kernel estimat...

A robust multimodel framework for ensemble seasonal hydroclimatic forecasts

We provide a framework for careful analysis of the different methodological choices we make when constructing multimodel ensemble seasonal forecasts of hydroclimatic variables. Specifically, we focus on three common modeling decisions: (i) number of models, (ii) multimodel combination approach, and (iii) lead time for prediction. The analysis scheme includes a multimodel ensemble forecasting algorithm based on nonparametric regression, a set of alternatives for the options previously pointed, and a selection of probabilistic verification methods for ensemble forecast evaluation. The usefulness of this framework is tested through an example application aimed to generate spring/summer streamflow forecasts at multiple locations in Central Chile. Results demonstrate the high impact that subjectivity in decision-making may have on the quality of ensemble seasonal hydroclimatic forecasts. In particular, we note that the probabilistic verification criteria may lead to different choices regarding the number of models or the multimodel combination method. We also illustrate how this objective analysis scheme may lead to results that are extremely relevant for the case study presented here, such as skillful seasonal streamflow predictions for very dry conditions.

Downscaling Climate Changes for Santiago: What Effects can be Expected?

This chapter describes the methodology used to analyse climate scenarios and their impact on hydro-meteorological variables in the Metropolitan Region of Santiago de Chile (MRS) and the results thereof. Using a downscaling methodology for future IPCC A2 and B1 scenarios (and B2 for stream flow), temperature, precipitation and secondary variable trends are estimated for the 2045–2065 time frame. The findings suggest that Santiago will be a drier and hotter city in the near future and have a high number of days with extreme temperatures. Lower precipitation rates are expected to lead to decreasing magnitudes in the stream flow of the two main rivers, Maipo and Mapocho, particularly in the summer months. Based on the data presented below, expected climate change impacts are analysed and adaptation needs identified for the MRS.

The influence of urban expansion on the flood hazard in Santiago de Chile

Land-use changes associated with urban expansion frequently lead to an increase of hazards, both natural and man-made [1], [2], [3], [4]. The process of urbanization is often associated with land-use and land-cover (LULC) changes, leading to a reduction of the infiltration capacity of newly urbanized soil. The goal of this study is to analyze the influence of urban growth on the flood hazard in the case of Santiago de Chile. Therefore, the hydrological model HEC-HMS (Hydrologic Engineering Center - Hydrologic Modeling System) is applied to model the hydrologic system at two time steps. The model allows balancing rainfall and runoff with respect to land use characteristics. A comparison with existing flood hazard maps created based on information of previous floods in Santiago can be used for validation. After successful modeling, i.e. representation of the functioning of the river ecosystem, alternative land-use scenarios can be created to predict the influence of urban land-use / land-cover changes on flood hazard. As a first step, the land-use patterns from 2002 and from a comparative date in 1993 have been delineated from high and very high resolution satellite data (Landsat/SPOT, ASTER) using pixel-based analysis. With this methodology, land-use changes of classes with common hydrological properties could be detected and a first quantification could be achieved. Measured rainfall and runoff data for the past 30 years have been analyzed to investigate the changes in precipitation and runoff characteristics. Finally, precipitation and runoff values are related to land-use changes applying the hydrological model. Applying this approach, the influence of land-use types and their spatial pattern on the runoff characteristics in a sub-catchment is investigated. The application of the hydrological model will help to develop scenarios that can be used to estimate the reaction of an urban environment to extreme rainfall and to suggest improved land-use planning in order to mitigate the flood hazard. Future land-use scenarios can be simulated with respect to their influence on the storm water behavior once the model is calibrated.

Patterns of glacier ablation across North-Central Chile: Identifying the limits of empirical melt models under sublimation-favorable conditions

We investigate the energy balance and ablation regimes of glaciers in high-elevation, dry environments using glaciometeorological data collected on six glaciers in the semiarid Andes of North-Central Chile (29–34°S, 3127–5324 m). We use a point-scale physically based energy balance (EB) model and an enhanced Temperature-Index (ETI) model that calculates melt rates only as a function of air temperature and net shortwave radiation. At all sites, the largest energy inputs are net shortwave and incoming longwave radiation, which are controlled by surface albedo and elevation, respectively. Turbulent fluxes cancel each other out at the lower sites, but as elevation increases, cold, dry and wind-exposed conditions increase the magnitude of negative latent heat fluxes, associated with large surface sublimation rates. In midsummer (January), ablation rates vary from 67.9 mm w.e. d−1 at the lowest site (∼100% corresponding to melt), to 2.3 mm w.e. d−1 at the highest site (>85% corresponding to surface sublimation). At low-elevation, low-albedo, melt-dominated sites, the ETI model correctly reproduces melt using a large range of possible parameters, but both the performance and parameter transferability decrease with elevation for two main reasons: (i) the air temperature threshold approach for melt onset does not capture the diurnal variability of melt in cold and strong irradiated environments and (ii) energy losses decrease the correlation between melt and net shortwave radiation. We summarize our results by means of an elevation profile of ablation components that can be used as reference in future studies of glacier ablation in the semiarid Andes.