Subject: Land Use Parameters : Rainfall-Runoff Method _ Soil Moisture Method Posted: 7/19/2016 Viewed: 1229 times
Dear WEAP community,

I have some questions with respect to land use parameters under the Rainfall-Runoff _ Soil Moisture Method, and would highly appreciate your answer for these questions.

(1) Under this method, (Soil Water Capacity) is defined as the effective water holding capacity of the upper soil layer. Given that the soil water holding capacity can be estimated at three points (Saturation, Field Capacity and Wilt point), how would the (Soil Water Capacity)/ (effective water holding capacity) be estimated from the previous values?

(2) (Root Zone Conductivity) is defined as the hydraulic conductivity rate at full saturation. In the case of a layered soil with different horizons, effective saturated hydraulic conductivity can be either calculated so as to represent horizontal hydraulic conductivity and vertical hydraulic conductivity. Based on WEAP notion, which value shall be taken as the saturated hydraulic conductivity to represent the land cover fraction? In other words, how it works with WEAP in case of layered soils to represent saturated hydraulic conductivity?

(3) Does WEAP apply a certain limit for the root zone or this has to be related to the type of crop or land cover being modelled to determine the soil horizons, for example, falling under this zone?

(4) Is there a certain method you would recommend to reasonably estimate or set preferred flow direction values?

Your answer is highly appreciated

Kind Regards

Stephanie Galaitsi

Subject: Re: Land User Parameters : Rainfall-Runoff Method _ Soil Moisture Method Posted: 7/20/2016 Viewed: 1188 times
Dear Mohammed,

The soil water capacity is meant as the top "bucket" of a two bucket model - the area that is impacted by the roots of the plants. Therefore the measurements can vary between different plant areas. So its capacity, as modeled in WEAP, does not correspond to any of the values you mentioned - it is a millimeter value that you can think of like the height of water that can be stored in that soil (or root) zone.

For root zone conductivity, you'll want to come up with a value that adequately represents both the horizontal and vertical conductivity - but remember that you as the user partition the flow between horizontal and vertical using the "Preferred Flow Direction" variable. So if your system has high vertical conductivity and low horizontal conductivity, you still write a single value for the "root zone conductivity", but give a number close to zero for the "preferred flow direction" (for the land use type under examination).

The root zone may vary between different land use types.

For recommending preferred flow directions, you could use the method I suggest above where you account for differences between horizontal and vertical flow.h

Mohammed Yassin

Subject: Re: Land User Parameters : Rainfall-Runoff Method _ Soil Moisture Method Posted: 7/21/2016 Viewed: 1178 times
Dear Stephanie,

Thanks very much for answering my questions.

With respect to the first question and answer, I understand now that the soil water capacity is a millimeter value which represents the height of water that can be stored in the soil. However, I am still confused on how to estimate this value.

Yates et. al. (2005), a journal paper describing WEAP model characteristics, mentioned that in the mass balance equation z1,j is given as a fraction of the total effective storage and varies between 0 and 1, and 0 represents the permanent wilting point and 1 field capacity. The total effective storage of the upper layer is approximated by an estimate of the soil water holding capacity (Swj in mm) prescribed for each land cover fraction, j.

In the user guide, however, it is indicated that Z1=1 refers to soil being saturated.

Some resources also refer to the soil water holding capacity as being the available water capacity (i.e. field capacity - permanent wilting point),and provide available capacities for different soil textures (i.e. in/ft of soil depth), and also field capacity in in/ft

If these values (soil water capacities at the three points) can be found in mm (taken in consideration only the area impacted by roots), would it be possible to estimate the water holding capacity parameter for this method (soil moisture) using these values, and what would be this parameter in term of the mm values of these variables, or there is a totally another procedure that has to be followed to estimate this!

For example, for a sandy loam soil of 0.65m depth with horizons of different particle sizes, the averaged values for saturation, field capacity, wilting and available are 21.70 , 15.60 , 5.93 and 9.64 %. This can be multiplied be the total depth to provide a mm value. Would the soil water capacity on the mass balance equation of this method be any of these values in mm (considerating the effective root depth), or 'water holding capacity' represents a value outside these!

I highly appreciate your further clarification to resolve this confusion.