H33A-01
Spatiotemporal Analysis of a Monsoon Flood Event in Northwestern Mexico: Insights from Remote Sensing and Hydrologic Modeling
Hydrological processes in the North American Monsoon (NAM) region are poorly understood due to the strong seasonal signature of the precipitation forcing and land surface conditions, including vegetation state and soil moisture, occurring in the mountainous, semiarid landscape. Mechanistic understanding can be achieved through field observations of hydrologic states and fluxes, remote sensing of changing land surface conditions and numerical modeling of distributed hydrological processes. A particularly promising approach to identifying the salient mechanisms is through analysis of flood events occurring after intense mesoscale storms. In this study, we present an analysis of a monsoon flood event generated in the Río San Miguel watershed in northwestern Mexico using a variety of observational and modeling tools. We integrate data sets obtained from a hydrometeorological network, satellite-based precipitation forcing and aircraft-based soil moisture estimates. A distributed hydrologic model, known as the TIN-based Real-time Integrated Basin Simulator (tRIBS), is applied to the large basin utilizing the hydrological observations and regional topography, soils and vegetation data sets. We focus a portion of our analysis on reconstructing the flood event and understanding the causative factors through use of the numerical model. We gain further insight on the hydrological processes by comparing the spatial pattern of the basin response to remotely-sensed observations of precipitation, vegetation and soil moisture fields. We also assess the role played by antecedent soil moisture conditions on the flood response. Finally, we discuss some avenues for improvements in hydrological modeling of NAM basins characterized by complex terrain and seasonal land surface conditions.
H33A-02
Seasonal Evolution of Land Surface Conditions in the North American Monsoon Region from MODIS Observations
Satellite remote sensing offers the opportunity to measure diverse landscape properties over large, remote regions. In addition, remote sensing permits describing and quantifying spatial and spectral patterns which can be correlated with ancillary information over multiple time periods. For example, the MODIS sensor, on board the TERRA and AQUA satellites, provides global coverage of land surface dynamics, such as changes in vegetation cover and surface albedo, at a high spatial and temporal resolution. These dynamic processes are poorly understood in environments experiencing a dramatic response to seasonal precipitation caused by large scale monsoon systems. In the North American Monsoon (NAM), for example, there is ample evidence that significant vegetation changes occur after the monsoon onset. In this study, we utilize remote sensing techniques to determine the seasonal evolution of land surface conditions in the Río Sonora basin in northwestern Mexico. We focus our efforts on determining the spatial and temporal changes in vegetation indices (NDVI, EVI, LAI), surface albedo and land surface temperature. The seasonal and inter-annual evolution of these distributed land surface conditions is currently unknown. Nevertheless, these variables may play a crucial role in land- atmosphere interactions, with possible influences on convective precipitation. Our study correlates the remotely- sensed variables with precipitation and soil moisture observations at 16 continuous stations in the river basin. We also discuss the intra- and inter-annual variations of land processes, soil moisture and precipitation within different ecosystems. Finally, we describe evidence of observed processes that are characteristic of a vegetation- precipitation feedback mechanism in the region.
H33A-03
Relationship Between Evapotranspiration and Precipitation Pulses in a Semiarid Rangeland Estimated by Moisture Flux Towers and MODIS Vegetation Indices
We used moisture Bowen ratio flux tower data and the Enhanced Vegetation Index (EVI) from the Moderate Resolution Imaging Spectrometer (MODIS) on the Terra satellite to measure and scale evapotranspiration (ET) over sparsely vegetated grassland and shrubland sites in a semiarid watershed near the Upper San Pedro River in southeastern Arizona from 2000 to 2004. The grassland tower site had higher mean annual ET (336 mm yr-1) than the shrubland tower site (266 mm yr-1) (P<0.001). ET measured at the individual tower sites was strongly correlated with EVI (r = 0.80 - 0.94). ET was moderately correlated with precipitation (P), and only weakly correlated with net radiation or air temperature. The strong correlation between ET and EVI, as opposed to the moderate correlation with rainfall, suggests that transpiration (T) is the dominant process controlling ET at these sites. ET could be adequately predicted from EVI and P across seasons and tower sites (r2 = 0.74) by a single multiple regression equation. The regression equation relating ET to EVI and P was used to scale ET over 25 km2 areas of grassland and shrubland around each tower site. Over the study, ratios of T to ET ranged from 0.75 to 1.0. Winter rains stimulated spring ET, and a large rain event in fall, 2000, stimulated ET above T through the following year, indicating that winter rain stored in the soil profile can be an important component of the plants' water budget during the warm season in this ecosystem. We conclude that remotely sensed vegetation indices can be used to scale ground measurements of ET over larger landscape units in semiarid rangelands, and that the vegetation communities in this landscape effectively harvest the available precipitation over a period of years, even though precipitation patterns are variably seasonally and interannually.
H33A-04
Evaluation of Systematic Errors in the Data From the NAME Rain Gauge Network
Simple tipping bucket rain gauges were chosen for the NAME Rain Gauge Network in order to provide the maximum geographic coverage and density with the available funds. Unfortunately, it is well known that these gauges tend to underestimate rainfall, especially for high intensities. A simple methodology was developed and tested to correct these values using rainfall aggregated over different time intervals. These corrections are generally fairly small, but could be large for extraordinary rainfall rates. They are easy to implement using the event data and improve the results when compared to monthly rainfall recorded at manually operated gauges.
H33A-05
Hydrological drought sensitivity to land use changes in the Yaqui River Basin
Land-use changes affect streamflow generation. Deforestation and agricultural intensification increase streamflow while afforestation reduces it. In semiarid basins, such as the Yaqui River basin (YRB) in Northwestern Mexico, changes in streamflow generation may impact the sustainability of the current agricultural practices in the region. Water resources in the YRB are influenced by different climate-phenomena, such as the North American Monsoon, El Niño Southern Oscillation (ENSO), and the Pacific Decadal Oscillation (PDO), and by anthropogenic activities. Here we will present an evaluation of the sensitivity of drought events in the YRB to land use changes under different climatological conditions. Periods were identified where the ENSO and the PDO influence the temporal variability of streamflow from 1949 to 1999, using wavelet analysis. The drought events, which were influenced by these oscillations, were identified using the runoff-percentil anomaly. Increments of the agricultural and afforestation practices were implemented as boundary conditions, and used by the Variable Infiltration Capacity model (VIC) to simulate the hydrological surface components of the YRB. Droughts during the non-monsoon months of La Nina and the warm phase of the PDO years were sensitive to increase in crop land-use in the northernmost part of the YRB. The rest of the basin observed an increase in runoff, which reduced the drought occurrence. Drought sensitivity to afforestation was higher during the monsoon months. In neutral years followed by El Nino and in years during the cold phase of the PDO, drought sensitivity to afforestation was the highest during the monsoon months.
H33A-06
Temporal variability of water fluxes and surface energy exchanges over a Low Deciduous Forest in the NAM region
In this study the seasonal and yearly variability of latent heat flux and surface energy exchanges over a Low Deciduous Forest (LDF) is investigated. The LDF is one of the most diverse and most contrasting ecosystems in Mexico. It can be found from the south of Sonora to the border of Mexico with Guatemala. In the present study, the CO2 fluxes over the LDF were analyzed in the period from June 2004 to December 2006. The CO2, latent and sensible heat, and momentum fluxes were measured using the Eddy covariance method. The Eddy covariance system consists of a 3-D sonic anemometer (CSAT3, Campbell Scientific), a gas analyzer (LI-7500, LI-COR), and diverse meteorological equipment controlled by a datalogger (CR5000, Campbell Scientific). The measures were made at 10Hz, collecting, storing and averaging the measurements every 30 minutes. The same type of cover is present in, at least, 3 km around the tower. The measurements show some variability in total rainfall (400-500 mm/year), average latent and sensible heat flux (30-35 and 65-72 w m-2, respectively), radiation balance (118-127 w m-2). Reflectances vary from 0.15 during the dry season to 0.05 after the rainy season and from 0.35 to 0.15 for the red and near infrared regions respectively resulting in NDVI values ranging from 0.3 to 0.85 with an abrupt change just after the monsoon starts accompanied by a decrease in surface temperature (from 329 to 300 K, during the day). Despite of these changes, albedo only varies marginally, from 0.12 to 0.09. This ecosystem presents a clear growing season corresponding to the rain season in the summer.
H33A-07
Assessing Runoff and Suspended Sediment Using SWAT Under Different Land Uses in Tapalpa, Jalisco, Mexico
Runoff and erosion from different land uses within a watershed is one of the biggest concerns in Mexico. Different land uses affect the overall watershed, which may be observed by the amount and duration of runoff, erosion, pollution, and water quality. To understand the complex combination of land uses and analyze the results in hydrologic approach, different models may be used. One of them is the SWAT (Soil and Water Assessment Tool) model. For the purpose to understand the hydrologic cycle and the sediment yields over the entire Tapalpa watershed the SWAT model was used in this study. The Tapalpa watershed had different land uses such as pine forest, oak forest, shrubs, grass, agriculture and urban areas. The entire watershed is 21,000 ha and was subdivided into five subwatersheds, each one had different land use distribution. In one subwatershed the SWAT model was calibrated using two years of recorded runoff and precipitation data. The subwatershed El Carrizal had good results for the SWAT calibration, water yield had a high R2 of 0.85. The average rainfall from the subwatershed had 913 mm. During the first year, the runoff had a value of 90 mm or 9% of the rainfall, and the second year was 20 mm or 3% of the rainfall. Most of the runoff from this subwatershed came as subsurface flow; other subwatersheds with larger agricultural areas produce more surface runoff and more sediment. The suspended sediment were low for El Carrizal, the values were 0.23 t ha-1 yr-1 and 0.05 t ha-1 yr-1 for the first and second year respectively but other subwatersheds had high values up to 17 t ha-1 yr-1 of suspended sediment. Inside forested subwatersheds roads and grazing are the main source of sediment and need more understanding about the magnitude of the effects. Roads, agriculture lands and grazing areas need to be studied in detail to find better relationships among runoff, erosion, water quality and land use in order to suggest better management practices.