Ask A Farmwest Climate Question

Question:  Can you please explain how the historic average moisture deficit is calculated on http://www.farmwest.com/climate/et

For how many years?  How is the number calculated? For example, in 2010 to 2014 in the months  May 1st and September 30, each year the moisture deficit is around 300 to 400, the difference between evapotranspiration and total precipitation between 40 to 300 but the historical average deficit is always around 60 to 80 mm. With other words, I don't see any correlation  of the historic average with one of those two deficit calculations.

Answer:  Moisture deficit is evapotranspiration minus effective precipitation. It’s explained in this link: http://www.farmwest.com/node/933   Historical average is calculated within the time period that you’ve selected (e.g., May to Sep) over all the years that a particular station has data (e.g., since 1970). It is just a quick comparison of what it is currently compared to historical average. ~ Stephanie Tam, P.Eng., Water Management Engineer, BC Ministry of Agriculture


Question:  I was wondering if one could loosely correlate degree days to crop production over the province of BC.  Realizing that just because you have a certain degree day does not mean that that location might be right for the crop, but crops wouldn't require different degree days in different locations of BC correct?

Answer:  Very loosely yes, but consider differences in rain, daylength, sunshine and crops. Some crops respond more than others to high temperatures. Also differences in tolerating low temps and differences in maturity. Better for comparing when other factors are controlled for. Where temperature is a main factor it will work better. (Dr. S. Bittman)


Question:  I need data on first and last frost free days for various locations in our area (Williams Lake).  Your degree days data is a good start to other info I need.  Any other suggestions?

Answer:  You need to obtain long term weather data for these locations and decide what temperature you consider to be critical for frost (depends on the crop since some crops tolerate temperatures below zero, and also noting that spring and fall critical temperatures are often different).  You will also need to decide what type of calcuation you will want - mean or probabilistic.


Question:  How can I obtain Degree Day Values based on 10 deg C for my Grape Vines? What is the relevance of publishing GDDs based on 0 deg C ie what commercial crops grow at these temps?

Answer:  Different crops have different baselines. We have now re-developed the calculator to allow a user to set their own baseline temperature.  You can select -5C to +10C.


Question:  I am trying to figure out the average annual evapotranspiration in the Salmon Arm area using your evapotranspiration calculator. Is it possible to access the data set for evapotranspiration to figure out what the typical monthly evapotranspiration is in the Salmon Arm area ?

Answer:  For design purposes we use an ET depending on the water storage capacity of the soil.  This is a daily Peak ET rate.

Peak Evapotranspiration Rates for Various B.C. Locations

 Maximum Soil Water Deficit (Depth of Water)
Location1 in (in/d)2 in (in/d)3 in (in/d)4 in (in/d)5 in (in/d)
Salmon Arm0.190.170.170.160.16

The annual crop water requirement is listed below. You need to divide by the efficiency of the irrigation system for the gross water required.

Ie. If using hand line divide by 0.75. 16in / 0.75 = 21.3 in of annual requirement.

Estimated Annual Crop Water Requirements for Various B.C. Locations

 Estimated Annual Crop Water Requirments
Location1 in (in/d)2 in (in/d)3 in (in/d)4 in (in/d)5 in (in/d)
Salmon Arm211613119

The data on Farmwest will only be for single year. The above numbers are base on 30 years of data.  They come from the BC Irrigation Management guide.

Farmwest is using May 1 - Sept 30 Moisture deficit (MD) for a grass crop

2007   MD 543mm   21.37 in
2008   MD 510mm   20.0 in
2009   MD 568mm   22.4 in

You can set the date for each month too.  For example, 2008 ... 

May   MD 96 mm
June  MD 133 mm
July   MD 128 mm
Aug   MD 72 mm
Sept  MD 81 mm