Menberu M. Bitew

Evaluation of High-Resolution Satellite Rainfall Products through Streamflow Simulation in a Hydrological Modeling of a Small Mountainous Watershed in Ethiopia

AbstractThis study focuses on evaluating four widely used global high-resolution satellite rainfall products [the Climate Prediction Center’s morphing technique (CMORPH) product, the Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis (TMPA) near-real-time product (3B42RT), the TMPA method post-real-time research version product (3B42), and the Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks (PERSIANN) product] with a spatial resolution of 0.25° and temporal resolution of 3 h through their streamflow simulations in the Soil and Water Assessment Tool (SWAT) hydrologic model of a 299-km2 mountainous watershed in Ethiopia. Results show significant biases in the satellite rainfall estimates. The 3B42RT and CMORPH products perform better than the 3B42 and PERSIANN. The predictive ability of each of the satellite rainfall was examined using a SWAT model calibrated in two different approaches: with rain gauge rainfall as input, and with each...

Evaluation Through Independent Measurements: Complex Terrain and Humid Tropical Region in Ethiopia

Evaluation of satellite rainfall products was conducted using ground-based daily rainfall measurements at 22 locations within a grid of 5×5 km collected during summer monsoon 2008 in a very complex terrain and humid tropical region in Ethiopia. Two high-resolution satellite rainfall products, namely, PERSIANN-CCS available at 1-h and 0.04° resolution, and CMORPH available at 30-min and 0.08° resolution. Both remotely-sensed products underestimated heavy events by about 50%, and so caution must be exercised when using CMORPH and PERSIANN-CCS as input for flood forecasting, as this could underestimate large flood events. The underestimation in monthly total rainfall was significant (32% for CMORPH, 49% for PERSIANN-CCS), and this error level needs to be acknowledged in applications that require monthly analyses. PERSIANN-CCS failed to detect half of the light events, and consistently those under 1.6 mm/day, indicating clearly that PERSIANN-CCS has difficulty detecting light rainfall events in complex terrain.

Accuracy of satellite rainfall estimates in the Blue Nile Basin: Lowland plain versus highland mountain

The demand for accurate satellite rainfall products is increasing particularly in Africa where ground-based data are mostly unavailable, timely inaccessible, and unreliable. In this study, the accuracy of three widely used, near-global, high-resolution satellite rainfall products (CMORPH, TMPA-RT v7, TMPA-RP v7), with a spatial resolution of 0.25° and a temporal resolution of 3 h, is assessed over the Blue Nile River Basin, a basin characterized by complex terrain and tropical monsoon. The assessment is made using relatively dense experimental networks of rain gauges deployed at two, 0.25° × 0.25°, sites that represent contrasting topographic features: lowland plain (mean elevation of 719 m.a.s.l.) and highland mountain (mean elevation of 2268 m.a.s.l.). The investigation period covers the summer seasons of 2012 and 2013. Compared to the highland mountain site, the lowland plain site exhibits marked extremes of rain intensity, higher mean rain intensity when it rains, lower frequency of rain occurrence, and smaller seasonal rainfall accumulation. All the satellite products considered tend to overestimate the mean rainfall rate at the lowland plain site, but underestimate it at the highland mountain site. The satellite products miss more rainfall at the highland mountain site than at the lowland plain site, and underestimate the heavy rain rates at both sites. Both sites have uncertainty (root mean square error) values greater than 100% for 3 h accumulations of <5 mm, or daily accumulations of <10 mm, and the uncertainty values decrease with increasing rainfall accumulation. Among the satellite products, CMORPH suffers from a large positive bias at the lowland plain site, and TMPA-RP and TMPA-RT miss a large number of rainfall events that contribute nearly half of the total rainfall at the highland mountain.

Interactions among hydraulic conductivity distributions, subsurface topography, and transport thresholds revealed by a multitracer hillslope irrigation experiment

Interactions among hydraulic conductivity distributions, subsurface topography, and lateral flow are poorly understood. We applied 407 mm of water and a suite of tracers over 51 hours to a 12 by 16.5 m forested hillslope segment to determine interflow thresholds, preferential pathway pore velocities, large-scale conductivities, the time series of event water fractions, and the fate of dissolved nutrients. The 12% hillslope featured loamy sand A and E horizons overlying a sandy clay loam Bt at 1.25 m average depth. Interflow measured from two drains within an interception trench commenced after 131 and 208 mm of irrigation. Cumulative interflow equaled 49% of applied water. Conservative tracer differences between the collection drains indicated differences in flow paths and storages within the plot. Event water fractions rose steadily throughout irrigation, peaking at 50% sixteen hours after irrigation ceased. Data implied that tightly held water exchanged with event water throughout the experiment and a substantial portion of pre-event water was released from the argillic layer. Surface-applied dye tracers bypassed the matrix, with peak concentrations measured shortly after flow commencement, indicating preferential network conductivities of 864 to 2240 mm/h, yet no macropore flow was observed. Near steady-state flow conditions indicated average conductivities of 460 mm/h and 2.5 mm/h for topsoils and the Bt horizon, respectively. Low ammonium and phosphorus concentrations in the interflow suggested rapid uptake or sorption, while higher nitrate concentrations suggested more conservative transport. These results reveal how hydraulic conductivity variation and subsurface topographic complexity explain otherwise paradoxical solute and flow behaviors. This article is protected by copyright. All rights reserved.

Are Satellite-Gauge Rainfall Products Better than Satellite-Only Products for Nile Hydrology?

The objective of this study is to compare the performances of two rainfall products (with resolutions of 3-h, 0.25°×0.25°) developed by the Tropical Rainfall Measuring Mission (TRMM) Multi-satellite Precipitation Analysis (TMPA) method: TMPA 3B42RT (a real-time version that does not include any rain gauge data) and TMPA 3B42 (a research version that combines TMPA 3B42RT with global rain gauge data). These products are separately used as input into the SWAT hydrological model to simulate daily streamflow for two adjoining watersheds (Koga with drainage area of 299 km2, and Gilgel Abay with a drainage area of 1,656 km2) in the Ethiopian part of the Nile basin, and the simulations are then compared to observed streamflow. Results turn the conventional notion on its head: the satellite-only TMPA 3B42RT products are found to be much better than the satellite-gauge TMPA 3B42 products in terms of their ability in reproducing daily streamflow. Nile hydrologist are advised to use TMPA 3B42RT over TMPA 3B42. Algorithm developers are advised to take a deeper look into their bias adjustment techniques especially in mountainous topography and rain gauge sparse regions.