Geology topics

Gabriel B. Senay

Publications and source records attributed to Gabriel B. Senay.

115 records · Page 7Linked to original sources

USGS2007

Remote sensing sensors and applications in environmental resources mapping and modeling

The history of remote sensing and development of different sensors for environmental and natural resources mapping and data acquisition is reviewed and reported. Application examples in urban studies, hydrological modeling such as land-cover and floodplain mapping, fractional vegetation cover and impervious surface area mapping, surface energy flux and micro-topography correlation studies is discussed. The review also discusses the use of remotely sensed-based rainfall and potential evapotranspiration for estimating crop water requirement satisfaction index and hence provides early warning information for growers. The review is not an exhaustive application of the remote sensing techniques rather a summary of some important applications in environmental studies and modeling.

Sensors↗

USGS2005

Climate science and famine early warning

Food security assessment in sub-Saharan Africa requires simultaneous consideration of multiple socio-economic and environmental variables. Early identification of populations at risk enables timely and appropriate action. Since large and widely dispersed populations depend on rainfed agriculture and pastoralism, climate monitoring and forecasting are important inputs to food security analysis. Satellite rainfall estimates (RFE) fill in gaps in station observations, and serve as input to drought index maps and crop water balance models. Gridded rainfall time-series give historical context, and provide a basis for quantitative interpretation of seasonal precipitation forecasts. RFE are also used to characterize flood hazards, in both simple indices and stream flow models. In the future, many African countries are likely to see negative impacts on subsistence agriculture due to the effects of global warming. Increased climate variability is forecast, with more frequent extreme events. Ethiopia requires special attention. Already facing a food security emergency, troubling persistent dryness has been observed in some areas, associated with a positive trend in Indian Ocean sea surface temperatures. Increased African capacity for rainfall observation, forecasting, data management and modelling applications is urgently needed. Managing climate change and increased climate variability require these fundamental technical capacities if creative coping strategies are to be devised.

Philosophical Transactions of the Royal Society B:↗

USGS2005

Evaluating MODIS data to estimate irrigated crop production in Afghanistan using a thermal-based ET fraction approach

Accurate crop performance monitoring and production estimation is critical for timely assessment of the food balance of several countries in the world. Recently, the Famine Early Warning System Network (FEWS NET) has been monitoring crop performance and to some extent relative production using satellite derived data and simulation models in Africa, Central America and Afghanistan where ground based monitoring is limited due to the scarcity of weather stations. The commonly used crop monitoring models use a crop water balance algorithm with inputs from satellite-derived rainfall. While these models provide useful monitoring for rain-fed agriculture, they are ineffective for irrigated areas. Over 80% of the agricultural production in Afghanistan is from irrigated agriculture. In this study, we implemented a thermal-based ET fraction approach to monitor and assess the performance of irrigated agriculture in Afghanistan using the combination of 250-m NDVI and 1-km Land Surface Temperature (LST) data from MODIS. Six images per year were used to estimate seasonal evapotranspiration (ET) from irrigated lands in a given growing season between 2000 and 2004. Seasonal ET estimates from the different years were used as relative indicators of year-to-year production magnitude differences. The results were comparable to field reports and crop water balance based estimates for irrigated watersheds in that 2003 was a good year for crop production in Afghanistan. The advantage of this method over crop water balance method is that it helps identify irrigated areas directly and thus helps estimate total irrigated area and its spatial distribution in a given region.

Conference Paper↗

USGS2005

Recent drought tendencies in Ethiopia and equatorial-subtropical eastern Africa

This report presents an analysis of Ethiopian rainfall from 1960 through 2004, based on 186 gauges in Ethiopia and 373 gauges from surrounding countriesi. Substantial post1997 declines in March-September rainfall have been observed in the northeast, southeast and southwestern portions of the country, a result corroborated by several independent data sourcesii. The observed rainfall reductions have been accompanied by an increase in millions needing food aid (Figure 1). The greatest percent decrease has occurred during the Belg (March-May) season, though Kiremt (June-September) rains have also diminished in many areas. This recent dryness may be linked to a warming tendency in the southern Indian Oceaniii, and is likely to impact densely populated and/or water-insecure regions of southern and eastern Ethiopia. These areas face declines in rainfall and increasing demands on land, water and agriculture. Recent dryness, dropping coffee prices, rising population and land degradation combine to create mounting destitution that must be countered with innovative planning and policiesiv. This report is organized as follows. Section 1 provides context for our results by summarizing recent trends in food aid, precipitation and the regional climate. Section 2 quantifies the water availability situation by analyzing potentially available runoff. Section 3 describes the results of our historical rainfall analysis. Section 4 relates the observed rainfall variations to circulation and sea surface temperature patterns in the Indian and Pacific Oceans. Section 5 briefly summarizes our results.

Equatorial-subtropical Africa↗

USGS2003

The collaborative historical African rainfall model: description and evaluation

In Africa the variability of rainfall in space and time is high, and the general availability of historical gauge data is low. This makes many food security and hydrologic preparedness activities difficult. In order to help overcome this limitation, we have created the Collaborative Historical African Rainfall Model (CHARM). CHARM combines three sources of information: climatologically aided interpolated (CAI) rainfall grids (monthly/0.5° ), National Centers for Environmental Prediction reanalysis precipitation fields (daily/1.875° ) and orographic enhancement estimates (daily/0.1° ). The first set of weights scales the daily reanalysis precipitation fields to match the gridded CAI monthly rainfall time series. This produces data with a daily/0.5° resolution. A diagnostic model of orographic precipitation, VDELB—based on the dot-product of the surface wind V and terrain gradient (DEL) and atmospheric buoyancy B —is then used to estimate the precipitation enhancement produced by complex terrain. Although the data are produced on 0.1° grids to facilitate integration with satellite-based rainfall estimates, the ‘true’ resolution of the data will be less than this value, and varies with station density, topography, and precipitation dynamics. The CHARM is best suited, therefore, to applications that integrate rainfall or rainfall-driven model results over large regions. The CHARM time series is compared with three independent datasets: dekadal satellite-based rainfall estimates across the continent, dekadal interpolated gauge data in Mali, and daily interpolated gauge data in western Kenya. These comparisons suggest reasonable accuracies (standard errors of about half a standard deviation) when data are aggregated to regional scales, even at daily time steps. Thus constrained, numerical weather prediction precipitation fields do a reasonable job of representing large-scale diurnal variations.

International Journal of Climatology↗

USGS2003

Estimating Meher crop production using rainfall in the 'Long Cycle' region of Ethiopia

Meher season crop production is well correlated with April-May rainfall in the Ethiopian 'long cycle' crop growing region. This relationship is used to estimate 2003 Meher small farmer gross production at 87 =B1 10 million quintals (8.7 million MT 2 =B1 1 million MT) of cereals, pulses and other crops, using data from the Central Statistical Authority (CSA). Assuming recent levels of commercial and food aid imports, Belg production and population growth, this estimate will produce a food deficit of about 23 million quintals (2.3 million MT) for 2003-04, smaller than the one experienced during 2002-03 (34 million Qt, or 3.4 million MT), but similar to the deficit experienced in 1997-98. A long-term negative rainfall trend in the southwestern highlands may be aggravating the situation. Increasing food requirements and decreasing precipitation point toward chronic food shortages in the near future.

FEWS NET Special Report↗

USGS2002

Evaluating the performance of a crop water balance model in estimating regional crop production

The comparison between a spatially distributed crop index and reported yield was evaluated. The crop index was generated by a crop water balance model that simulates the reduction of crop yield (as a percentage of the potential) due to water deficit. This model is currently operational as a monitoring and forecasting tool for region-wide food security analyses in drought prone countries in Sub-Saharan Africa. The most important input parameters of the model are Africa-wide satellite-based rainfall estimate images and spatially distributed potential evapotranspiration images calculated from 6-hourly numerical weather model data. District level crop yield data from Ethiopia were used to evaluate the performance of the model. Historical sorghum yield data from 1996-1999 were used to evaluate the performance of a seasonal water requirement satisfaction index (WRSI) for sorghum. WRSI values and reported district yield data were significantly correlated. The WRSI model was particularly successful in capturing the response of the crop during a relatively dry year. In districts that never experienced water deficit during the study period, it was possible to infer the magnitude of yield variability that was caused by factors other than water supply.

Sub-Saharan Africa↗