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Journal of Arid Land  2020, Vol. 12 Issue (6): 950-963    DOI: 10.1007/s40333-020-0101-y
Research article     
Spatial regression approach to estimate synthetic unit hydrograph by geomorphic characteristics of watersheds in arid regions
Department of Civil and Architectural Engineering, College of Engineering, Sultan Qaboos University, Oman
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Rainfall-runoff relationship in arid regions is unique and challenging to study. Studies for bridging the hydro-meteorological knowledge gap for planning, designing and managing water resources is therefore vitally important. The objective of this study is to develop a method for estimating unit hydrograph at reasonably finer time resolutions (10-min and 1-h) which can be easily adaptable by practitioners at sub-catchment levels, especially when the focus area is ungauged. Observed wadi-flow at 5-min interval and tipping bucket rainfall measurements at 1-min interval were obtained to cover 10 major watersheds in Oman. The deconvolution method was applied to derive the unit hydrographs (UHs) from wadi-flow and excess rainfall. Key catchment characteristics such as the watershed area, length of the main wadi and the length to the centroid of the catchment area were derived from digital elevation model (DEM) data. The whole study area was then divided into 515 sub-catchments with various shapes and sizes. A strong relationship was found between the wadi length and the length to the centroid of the catchment area (R2>0.89). This relationship was then adopted to simplify the classical Snyder method to determine UHs. Moreover, several parameters of the Snyder method were calibrated to the arid environment by matching the peak-flow, lag-time and three time-widths (75%, 50% and 30% of the peak-flow) of 10-min and 1-h UHs with physical characteristics of the watersheds. All developed relationships were validated with independent rainfall and wadi-flow events. Results indicate that the calibrated parameters in these arid watersheds are quite distinct from those suggested for other regions of the world. A marked difference was found between the 10-min UHs estimated by the S-hydrograph method and the deconvolution method. Therefore, it is concluded that a method depends on natural hydro-meteorological conditions would be more practical in arid region. The proposed methodology can be used for water resources management in arid regions having similar climate and geographical settings.

Key wordsdeconvolution method      S-hydrograph method      Snyder method      DEM data      river length     
Received: 03 December 2019      Published: 10 November 2020
Corresponding Authors: N GUNAWARDHANA Luminda     E-mail:
About author: *Luminda N GUNAWARDHANA (E-mail:
Cite this article:

Luminda N GUNAWARDHANA, Ghazi A AL-RAWAS, Mahad S BAAWAIN. Spatial regression approach to estimate synthetic unit hydrograph by geomorphic characteristics of watersheds in arid regions. Journal of Arid Land, 2020, 12(6): 950-963.

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Fig. 1 Delineated catchments and locations of rain gauge stations (a) and Thiessen Polygons for Aday catchment (b)
Watershed Wadi gauge Area (km2) Wadi length (km) Length to the centroid (km)
Aday FB505467AD 794.2 54.6 25.2
Afi EL895407AD 313.0 34.2 16.3
Al Khawd FB104840AD 1660.2 91.9 39.6
Al Uqq FA160968AD 132.1 22.5 8.2
Arabiyin GA057335AD 303.8 37.7 18.9
Hayfadh FA777631AD 122.2 21.7 7.4
Lansab FA395799AD 44.7 13.3 8.5
Manzariya FA585595AD 206.4 28.3 9.8
Mayh FA596055AD 222.0 34.0 15.5
Miglas FA877343AD 556.8 55.5 25.7
Awabi EL574613AD 254.4 35.4 14.4
Jannah FA580672AD 130.8 29.9 18.9
Table 1 List of stations and delineated catchment characteristics
Fig. 2 Calibrated and validated relationship between L (the main wadi length from the most upstream point of the watershed to the catchment outlet) and Lc (the distance from the catchment outlet to the point along the main wadi which is the closest to the centroid of the watershed)
Fig. 3 Observed and simulated wadi-flow using 1-h UH (unit hydrograph)
Fig. 4 Comparison of 10-min UHs developed by two methods for Aday catchment
Fig. 5 Observed and simulated 10-min averaged peak-flows by two methods for all watersheds
Fig. 6 Relationship between peak-flow (Qp)and physical characteristics of the catchment derived using the optimal m value (0.7) for 1-h and 10-min UHs (unit hydrographs)
Fig. 7 Calibration of Ct (Equation 2) for estimating lag-time for the 1-h and 10-min UHs
Fig. 8 Time width at 75% (W75), 50% (W50) and 30% (W30) of the Qp for 1-h and 10-min UHs. A, catchment area.
Fig. 9 Comparison between developed 10-min UHs for Afi watershed with two different width-ratios
Fig. 10 Comparison of observed and simulated wadi-flows from proposed empirical equations
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