The HELP model presumably is the world wide most widely used model to estimate the water balance of cover and bottom liner systems for landfills and contaminated sites. Primary purpose is to assist design engineers and permit writing authorities in the comparison of design alternatives as judged by the water balance. The original HELP model was developed since 1982 funded by the U.S. Environmental Protection Agency. For the current version HELP 3.06/3.07 [6] a validation study was funded by the German Federal Ministry of Education and Research. It comprises a methodological, a theoretical and a practical part [1,2,3]. In the practical part, besides sensitivity analyses for topsoil layers and drainage layers an operational validation (output comparison) was performed using data of the water balance test fields on the landfill Hamburg-Georgswerder [5]. Each test field is 50 m long and 10 m wide, three are 4 % inclined and three 20 %. All fields have a 75 cm thick topsoil with grass vegetation, below follows a 25 cm thick drainage layer. There are three types of liners: a) compacted soil liner, 60 cm thick; b) composite liners with a HDPE geomembrane on a compacted soil liner acc. to a); c) extended capillary barrier. The main validation results are given below. Based on the results of the validation study and further investigations enhanced versions of HELP were developed correcting errors and enhancing or replacing submodels [3]; current version is HELP 3.80 D [4]. Precipitation: The best matches between measured and simulated data were obtained with precipitation data with systematic measurement errors in the level typical for the measurement procedure of the German Weather Service for fairly sheltered stations (about 10 %). Frozen soil (FS): The FS submodel is very simple and leads especially to too long phases with frozen soil, resulting in errors in surface runoff (much too high due to rain and snow melt on frozen soil) and lateral drainage (extreme peaks). The FS submodel was pragmatically enhanced in HELP-D. Surface runoff (SR): Measured runoff was very small (in the order of 1 mm/yr). SR is in general hard to estimate by a model like HELP working with daily precipitation data because SR largely depends on the rainfall intensity. SCS curve numbers (CN) calibrated on yearly measured runoff were 6 to 34 larger than the CN computed by HELP. Actual evapotranspiration (ETa): Level and seasonal distribution were well estimated by HELP. However, HELP 3.07 tends to underestimate the influence of the stand of vegetation and of soil properties on ETa; the range of yearly sums of ETa is too small. The ETa and the vegetative growth and decay submodels were corrected and enhanced in HELP-D. Lateral drainage (LD): LD largely depends on the inflow into the drainage layer that means esp. on ETa. Its level and seasonal distribution were well reproduced by HELP. Percolation through compacted soil liner (CSL): Deterioration of CSLs e.g. due to desiccation and shrinkage and preferential flow through macropores [5] are not considered in HELP. Thus, HELP may largely underestimate the percolation through CSLs. Furthermore, HELP 3.80 D allows the user to change soil and vegetation properties in a simulation run. References: [1] Berger, K. (1998) Validierung und Anpassung des Simulationsmodells HELP zur Berechnung des Wasserhaushalts von Deponien für deutsche Verhältnisse. Umweltbundesamt, Berlin. [2] Berger, K. (2000) Validation of the hydrologic evaluation of landfill performance (HELP) model for simulating the water balance of cover systems. Environmental Geology 39 (11), 1261-1274. [3] Berger, K. (2002) Potential and Limitations of Applying HELP Model for Surface Covers. Practice Periodical of Hazardous, Toxic and Radioactive Waste Management, Vol. 6, No. 3, 192-203 [4] Berger, K. et al. (2004) Das Hydrologic Evaluation of Landfill Performance (HELP) Modell. HELP 3.80 D. CD with program and electronic documents, Institute of Soil Science, University of Hamburg. http://www.geowiss.uni-hamburg.de/i-boden/fsimhelp_e.htm [5] Melchior, S., Berger, K., Vielhaber, B., Miehlich, G. (1994) Multilayered landfill covers: field data on the water balance and liner performance. In: Gee, G.W., Wing, N.R. (eds.) In-situ remediation: scientific basis for current and future technologies. Thirty-Third Hanford Symposium on Health and the Environment, 7-11 November 1994, Pasco, Washington, USA, 411-425. [6] Schroeder, P.R. et al. (1994) The Hydrologic Evaluation of Landfill Performance (HELP) Model. 2 Volumes: User's Guide for Version 3. EPA/600/R-94/168a; Engineering Documentation for Version 3. EPA/600/R-94/168b, US Environmental Protection Agency, Cincinnati, Ohio http://el.erdc.usace.army.mil/products.cfm?Topic=model&Type=landfill