AbstractThe jet erosion test (JET) is a widely applied method for deriving the erodibility of cohesive soils and sediments. There are suggestions in the literature that further examination of the method widely used to interpret the results of these erosion tests is warranted. This paper presents an alternative approach for such interpretation based on the principle of energy conservation. This new approach recognizes that evaluation of erodibility using the jet tester should involve the mass of soil eroded, so determination of this eroded mass (or else scour volume and bulk density) is required. The theory partitions jet kinetic energy flux into that involved in eroding soil, the remainder being dissipated in a variety of mechanisms. The energy required to erode soil is defined as the product of the eroded mass and a resistance parameter which is the energy required to entrain unit mass of soil, denoted J (in J/kg), whose magnitude is sought. An effective component rate of jet energy consumption is defined which depends on depth of scour penetration by the jet, but not on soil type, or the uniformity of the soil type being investigated. Application of the theory depends on experimentally determining the spatial form of jet energy consumption displayed in erosion of a uniform body of soil, an approach of general application. The theory then allows determination of the soil resistance parameter J as a function of depth of scour penetration into any soil profile, thus evaluating such profile variation in erodibility as may exist. This parameter J has been used with the same meaning in soil and gully erosion studies for the last 25 years. Application of this approach will appear in a companion publication as part 2. Copyright © 2017 John Wiley & Sons, Ltd.