Flow in streams of arid and semi-arid regions is not perennial and usually floods occur after seasonal

precipitations. Therefore, the bed material is dry and underneath soil is in unsaturated condition prior to

occurrence of floods. As a result, volume of flood hydrographs decreases due to infiltration as they travel

downstream. This reduction in hydrograph volume is usually termed as transmission losses. Because of the

existence of ephemeral rivers in most parts of Iran, study on transmission losses in these areas is necessary.

In this paper, regression equations are developed for estimating transmission losses in ephemeral rivers of

Ghara-Ghoom catchment in Khorasan Razavi province. To achieve this, gauging stations' data and physical

characteristics of Kashafrud, Kardeh and Zoshk rivers were collected and analyzed. Transmission losses

in the first kilometer was considered as dependent variable in regression analysis and its dependency

was tested against upstream flow volume, flow duration, antecedent moisture content, hydraulic conductivity,

slope and width of the river as independent variables. Upstream flow volume was found to be the

most important variable in explaining transmission losses. Three regression equations (linear and nonlinear)

were developed for relating transmission losses to the most significant involved variables for Ghara-

Ghoom catchment. Estimates of transmission losses can be made for any unit length (km) by calculating

the flow volume at the beginning of the unit length as flow volume minus the transmission loss in the previous

unit length. Such equations can be used by water resources managers and hydraulic structure designers