Available Water Holding Capacity and Available Water Storage Capacity

Hello Professor Krzic and Professor Brown,

I just have two questions about the two terminologies, Available Water Holding Capacity (AWHC) and Available Water Storage Capacity (AWSC).

1. I think the difference between AWHC and AWSC is that AWHC is the total amount of water available to a plant growing in a field soil. But AWSC is the water retained in soil between field capacity and permenat wilting point. So they are completely two different things. Is this correct? 2. In the lecture, you mentioned that the unit of field capacity is m3/m3. For instance, the field capacity in problem set 2 Question 1, 0.22 m3/m3. But in textbook, it is said that the unit of AWHC is cm. I understand the fuctions to calculate AWHC and AWSC are different. However, my question is, why AWHC and AWSC have different units, even thought they both are talking about capacity?

I hope I explained my question clearly. Please let me know if I make you confuse. :)

Thank you so much for answering my questions. I hope you will have a wonderful holiday!

Regards, Jing (Alice) Miao

Alice, be careful with the terminology.

Available water storage capacity, available water holding capacity and plant available water are the same. How is AWSC as defined in your class notes determined? (see lecture notes for Qualitative description of soil wetness - Jan 26)

Be clear to distinguish AWSC from maximum retentive capacity - when all the pores are filled with water (see also Jan 26 lecture notes).

re: units Soil moisture can be given on either at mass basis kg water / kg soil

OR a volumetric basis m3 water / m3 soil

whether you use m3/m3 or cm3/cm3 is simply a matter of unit conversion - see lab tips for help on unit conversion if you are unsure

Hello Professor Brown,

Thank you so much for answering my question.

Before I thought all these three terminologies had the same meaning. However, in the third edition textbook page 161 BOX 5.3, it said that the total amount of water available to a plant growing in a field soil can be estimated from the rooting depth of the plant and the amount of water held between field capacity and wilting percentage in each of the soil horizons explored by the roots. It use the function AWHC= (θmFC-θmWP)*Db*Dw*L. The unit they got is cm.

If Available water storage capacity, available water holding capacity and plant available water are the same, I was thinking why textbook has a different function to calculate AWHC with unit cm rather cm3/cm3? Did I miss something and interpret the textbook wrongly?

Thanks a lot!

Regards, Alice

Alice you are correct that the text is distinguishing the water available to the plant per rooting depth.

Fundamentally they are very similar except:

In the lecture notes we are distinguishing Available water storage capacity AWSC = FC - PWP [cm3 water / cm3 soil]. The assumption is made that water is applied evenly over space i.e. per cm2 (similar to the idea of rainfall; we report rainfall in mm or cm not cm3] Thus if we know the cm of water per cm of soil (vertically), we can multiply by the rooting depth (in cm) to get the cm of water available to the plant.

re: Box 5.3 (or Box 5.2 in the 2nd edition) - Calculation of the total amount of water available to a plant growing in the field

AWSC = FC - PWP

If moisture content is in g/g (mass basis) i.e. mg water per mg soil, you need to then convert this value into a volumetric water content (cm3 water / m3 soil). To do this you multiply by the ratio of the bulk density of the soil / density of water.

Once you have a volumetric water content (cm3/cm3), to determine the amount of water in the rooting zone, simply multiply by the rooting depth (20 cm in your example) = AWC (in the rooting zone)

I like being very clear with my units, i.e. cm water / rooting zone