IPCC AR5, the representative concentration pathways

One of the most important events expected for this 2014 is the official release of the Intergovernmental Panel on Climate Change (IPCC) fifth assessment report (AR5). The IPCC began working on the AR5 in 2010. Between 2011 and 2013 some drafts were published on the internet. In September 2013 the working group 1 (The physical science basis) completed its report.The working group 2 (Impacts, adaptation and vulnerability) and the working group 3 (Mitigation of climate change) are expected to finish this year aloing with the nfinal synthesis report. but by this years it is expected that the final official version to be released.

Up to the last year we used to talk about the Special Report on Emissions Scenarios (SRES), but the AR5 uses a new term: Representative Concentration Pathway (RCP).

AR5 relies on the Coupled Model Intercomparison Project Phase 5 (CMIP5), an international effort among the climate modeling community to coordinate climate change experiments. Most of the CMIP5 and Earth System Model (ESM) simulations for AR5 were performed with prescribed CO2 concentrations reaching 421 ppm (RCP2.6), 538 ppm (RCP4.5), 670 ppm (RCP6.0), and 936 ppm (RCP 8.5) by the year 2100. (

Image 1. Cover of the IPCC-AR5

Source: Sciencemediacentre

What is the RCP

They are prescribed pathways for greenhouse gas and aerosol concentrations, together with land use change, that are consistent with a set of broad climate outcomes used by the climate modelling community.

The pathways are characterised by the radiative forcing produced by the end of the 21st century. Radiative forcing is the extra heat the lower atmosphere will retain as a result of additional greenhouse gases, measured in Watts per square meter (W/m2).

Image 2. Temperature differences by the end of the 21st century

Source: Zeeburgnieuws

RCP vs SRES

The RCPs span a wider range of possibilities than the SRES marker scenarios used in the modelling for the IPCC 3rd and 4th Assessment.

RCPs start with atmospheric concentrations of greenhouse gases rather than socioeconomic processes. This is important because every modelling step from a socioeconomic scenario to climate change impacts adds uncertainty. By starting with concentrations, there are fewer steps to impacts and therefore less cumulative uncertainty in impact assessments. This way uncertainty is shared more evenly among the various components. RCP also includes mitigation and adaptation policies. The figure shows a comparison of the CO2 projections according to SRES and RCP.

Image 3. Relation between RCO and SRES scenarios

Source: WMO

References & Further Reading
IPCC 5th assessment report
Representative concentration pathways

Boundary conditions in HEC RAS

The basic data needed to calculate hydraulic profiles of a channel are the discharge, the channel geometry, the water elevation at a control section and the channel roughness.

Discharge is a given data and the channel geometry is obtained by measurements. Usually the water elevation at control section (boundary condition) and channel roughness are unknown and have to be estimated by indirect ways.

Although there are different approaches and recommendations for defining the boundary conditions, they are just suggestions and incorporate an error to the vicinity of the boundary station and cross sections.

The best approach is to perform a sensitivity analysis of the boundary condition, and if needed to perform an uncertainty analysis. Although the task of updating either the boundary condition or manning roughness are simple ones, when repeating many times (sensitivity and uncertainty analysis) it may become a tedious and time consuming task; besides, the possibility of accidental errors when typing values increase.

Boundary conditions in HEC-RAS

A boundary condition defines the starting water level at the end of the river, which is needed so that the program can begin the calculations. HEC-RAS allows specifying boundary condition in one of the following options:

• Known water surface: This is the known water surface for the given profile. This option applies for cases where the water level was measured for a given discharge.
• Critical depth: With this option the program calculates the critical depth for the section and uses it as boundary condition. This option applies for cases where there is a control structure such as weir, gate or drop that controls and forces the critical depth.
• Rating Curve: In this option the water level is interpolated from the given rating curve. Usually this case applies for control station where water levels and discharges are measured constantly.
• Normal depth: In this option the program uses the energy slope to calculate the normal depth with Manning's equation.

The most used boundary condition is the normal depth. Although the normal depth is unknown, usually it is approximated either by using the channel slope or the water surface slope near the boundary station. Nevertheless, these are just approximations that incorporates an error to the vicinity of the boundary station; thus, is suggested to increase the cross sections and channel length (which increases the topographical data required) so that these errors do not affect the study area.

Figure 1 shows a river hydraulic profile under normal depth boundary condition located downstream. The hydraulic profile is from a river with a low topographic gradient around 0.0015 m/m. Different values of normal depth slope between 0.001 m/m and 0.015 m/m were tested. The uncertainties due to the boundary condition propagate some 1500 m upstream. The hydraulic profile in those 1500 m is strongly influenced by the selected slope.  

Image 1. Influence of boundary conditions

Source: AHYDRA

The best approach is to perform a sensitivity analysis of the boundary conditions, and if needed to perform an uncertainty analysis. In the present case, if the study area is located more than 1500 m upstream, then the uncertainties of the boundary condition have minor influence. However, if the study area is within the first 1500 m, then it is important to analyze the different profiles. For instance, a visual analysis shows that the highest slope forces a higher velocity and lower depth at the boundary section; thus, the backwater shows a concave curve to fit the imposed condition. On the other side, the other slopes shows a backwater profile with the same pattern over the whole river length. 

Although the task of updating the boundary condition is a simple one, when repeating many times (sensitivity and uncertainty analysis) it may become a tedious and time consuming task. For such purpose, we developed the Automating Hydraulic Analysis (AHYDRA) tool. This software allows updating boundary condition, executing the simulation and viewing the results from a single screen and a single instruction (button click).

Automating Hydraulic Analysis in HEC-RAS AHYDRA

AHYDRA is a freeware application that can be downloaded from its website.

It comes in a compressed .rar file. Just decompress it in the path “C” so that it will create a folder with the path “C:\AHYDRA” and launch it by double clicking the AHYDRA.exe icon. Before executing it, be sure that you have HEC-RAS version 4.1 installed. If you have a previous version the AHYDRA will not work.

NOTE: It is important to have the “C:\AHYDRA” path in order to overcome accessibility limitations with the “C:” and “Program Files” folder.

The following video shows the use of AHYDRA for automating boundary conditions in HEC RAS.

Video 1. Automating boudary conditins in HEC RAS

Source: AHYDRA

References & Further reading
HEC-RAS evolution
HEC RAS hydraulic reference
Open Channel hydraulics
AHYDRA basic tutorial
Automating hydraulic analysis AHUDRA
Youtube tutorials