Quoted from: Demissie, Misganaw, Abiola A. Akanbi, and Abdul Khan. Hydrologic Modeling of Landscape Functions of Wetlands. Illinois State Water Survey, Champaign, Research Report 125, 1997. https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.534.3382&rep=rep1&type=pdf 

      In an attempt to include the effects of soil, vegetative, and land-use characteristics of the watershed in the hydrologic model framework, the U.S. Department of Agriculture (USDA) Hydrograph Laboratory-74 (USDAHL-74) model was conceived (Holtan et al., 1975). It was developed in Maryland at the Agricultural Research Service (ARS)-USDA. This conceptual, lumped-parameter, continuous model uses variable time steps and is applicable for small, agricultural watersheds. Its considerable data requirements include continuous precipitation records, weekly temperatures, and pan evaporation, as well as data on soils, vegetation, landuse, and cultural practices.

      The general structure of the USDAHL-74 model is shown in figure 2. This model ignores interception loss on the assumption that it is small in comparison to total rainfall. Evapotranspiration is computed on the basis of the plant growth index, pan coefficient, and pan evaporation data. Infiltration is modeled by using the modified Holtan method (ibid.). Overland flow is simulated by a linear reservoir. Interflow (quick subsurface flow) is approximated by a linear exhaustion function based on the soil moisture depth in excess of the available soil water capacity. Ground-water flow and channel routing are both modeled as exponential reservoirs. 

Figure 2. General structure of the U.S. Department of Agriculture Hydrograph Laboratory-74 (USDAHL-74) model (after Holtan et al., 1975)