You may use HEC-HMS and HEC-RAS for the purpose. These models are freeware and downloaded from US army corps of engineers. We are in the process of applying these models to a hilly watershed. Hope we can give you an update in the near future.

QUAL2K, which is a freeware software and is probably the most widely used one can be used and requires data about the basic characters of the waterbody, including the slope.

PHREEQC (freeware) is a very flexible program that can simulate the chemistry of water in equilibrium with air and suspended materials. You would though have to make some assumptions on how to represent the physical characteristics of the system, e.g. residence times etc. or simply combine/compare this with results from another model, such as the one mentioned above.

This really depends on your objectives. Why modeling? for process understanding or predictions?

There are many models (SWAT, HPSF, MIKE SHE) that are can do the land phase of the watershed hydrologic cycles. QUAL2K is mostly for the water body phase.

For watersheds with steep slopes, the challenges is to make sure the flow pathways are described reasonably. In addition, ET is important as well if you want to model continuous flow and chemistry, and estimate water balances.

Please have a look at our Blue Model as described in the publication by Bach and Ostrowski: Analysis of intensively used catchments based on integrated modelling. Journal of Hydrology 2012, which simulates both quantity and quality in small catchments

You may use HEC-HMS and HEC-RAS for the purpose. These models are freeware and downloaded from US army corps of engineers. We are in the process of applying these models to a hilly watershed. Hope we can give you an update in the near future.

QUAL2K can be used for flow and the transport of particulates within the channel. It has a bunch of assumptions ( 1-D, well mixed etc which can be the situation in steep catchments with turbulent flow).

http://www.epa.gov/ATHENS/wwqtsc/html/qual2k.html

For runoff into the channel /stream, one can use models such as suggested by Ge Sun, as well as the open source modeling program WEAP, which can then link to QUAL2K.

http://www.weap21.org/webhelp/water_quality_constituents_with_qual2k.htm

These models are quite efficient in simulating discharge and water quality also. For simulating discharge rainfall, ET, Temp and other meteorogical parameters are used as input. Similarly you have to look at the most influencing parameters that effecting water quality for simulation.

There are quite a lot of responses. I am more interested in modeling the discharge in small hill streams (maybe 2-3 order streams). What I like to do is to find some relationship between hydrological processes and ecosystem in small pristine hill streams. I also interested in water quality since they might also be a possible pathway that ecosystem reflect to the changing hydrology. So, one key question for all these model is that how accurate are they, and how much discharge data do I need to calibrate and validate these models.

Hi Yang Pan

Please refer to the following paper which elaborates the suitability of HEC-HMS for slow simulation. We have shown that HEC-HMS can be used to simulate the flow with much accuracy.

D. Halwatura and M.M.M. Najim, 2013. Application of the HEC-HMS model for runoff simulation in a tropical catchment, Environmental Modelling & Software, 46: 155–162

Thank you Dr. Najim. This paper is very useful. One big problem for me is that we do not have so much data to calibrate our models. Currently we only have 20 years of flow data of a 2nd order stream, while we would like to simulate the flow data of nearly 20 small streams.

We also have the flow data of the downstream major river, is it possible to calibrate the model with the downstream station data and use this model to simulate the upstream discharge?

Yang, modeling 20 streams in ungaged watersheds is a challenge to anyone. The main challenge is not the model, but the preparing input data for climate, soils, vegetation characteristics. In addition to the models recommended by colleagues, the TOPMODEL used by USGS and others may also serve well for small hilly vegetated watersheds. I suspect all models will fail without good input data for those hilly watersheds that are driven by topography of land surfaces and the bedrock as well. Also, ET, a major flux, can be tricky to model for watersheds with complex terrain and land cover patterns.

may be you try to apply Artificial Intelligence such as ANN or SVM. It could be a good modeling tool for your study

would ANN require large amount of empirical data to 'train' the model for prediction purposes? the problem is Yang does not have the measured data to begin with. I would suggest Yang to measure the 21-streams' chemistry - hard labor but more reliable than modeling from no nothing.

If you do not have runoff data available for model calibration you need a process-based model that is capable to do a forward modelling of runoff by means of adequate input data (forcing data, topography, vegetation, soil properties, lower boundary etc.). This is even more important in case you want to do water quality simulation. Please find attached a related paper. although not new it hopefully gives some useful information on this topic.

I suggest you Hydrogeosphere (http://www.aquanty.com/). It is a process based model that is capable to simultaneously solve the equations for variably saturated subsurface flow and overland flow. In addition it is also capable to solve transport equations. It has a parallel model structure so you can run it on multi -processor machines. It is very wide-spread in the hydrologic/hydrogeological community and lot of modelling studies used Hydrogeosphere on alot of different topics. Unfortunately it is not freeware.

I would recommend RHESSYs.

You can even try with QUAL2E freely downloadable software. Even I am trying to looking into details of this software

I forgot to mention SWMM, probably the most widely used model for small urbanising catchments. The link below also gives you access to all other USEPA models:

http://www.epa.gov/athens/wwqtsc/html/swmm.html

EPA does an excellent job providing well documented open software for many different approaches to environmental modeling

i would recommand to you the SWAT model, one of the most complete hydrological model, you can simulate simultaniously , discharge, sediment, nutrients, pesticides, etc. and it is a free model.

http://swat.tamu.edu/

♦ I would recommend you trying WetSpa, a simple and parsimonious model, yet physical based that simulates hydrological processes of precipitation, snowmelt, interception, depression, surface runoff, infiltration, evapotranspiration, percolation, interflow, groundwater flow, etc. continuously both in time and space, for which the water and energy balance are maintained on each raster cell.

DEM, landuse and soil maps along with weather data are main input data to the WetSpa model.

The link attached below is an example of the model application:

"WetSpa model application in the Distributed Model Intercomparison Project (DMIP2)"

♦ You can also couple the model with an water chemistry model.

WetSpa Model Manual:

http://www.vub.ac.be/WetSpa/downloads/WetSpa_manual.pdf

Article WetSpa Model Application in the Distributed Model Intercompa...