how dry am i?

This post may be mainly for the math nerds among you, but I think it could be inter­est­ing to any gar­den­ers liv­ing in drought-prone parts of the world.

In my last post I men­tioned that I’d used instruc­tions in Olivier Filippi’s The Dry Gar­den­ing Hand­book to fig­ure out the drought stress index, or hydric deficit, for where I live in San Diego.

USDA zones are useful for dealing with minimum temperatures. For gardeners in the western U.S., Sunset zones provide more finesse, combining temperature with other climate conditions. The the drought stress numbers, however, are useful if you want to concentrate on understanding how many months a plant might be subjected to severe drying conditions due to lack of rainfall.

Filippi writes in his book that "everyone's drought is different," so be sure to consider factors other than this single number, things like total rainfall, humidity, the sun exposure a plant might get or the amount of wind your site experiences. The technique presented in The Dry Gardening Handbook derives from work of plant geographer Henri Gaussen.

Figuring out hydric deficit is pretty straightforward but will take a few minutes of your time. Either use a spreadsheet program like Excel or a sheet of paper. First, go to a site like World Climate where you can find your area's monthly total rainfall and monthly average temperatures. On the spreadsheet or paper set up a column with the months of the year, January to December. Next fill in a column with the monthly average rainfall in millimeters, and another column with the average monthly temperature in degrees Celsius.

Now you have two options: Fol­low the instruc­tions in the book, which isn’t that hard but requires mak­ing a graph with three dif­fer­ent axes. Or use my sim­pli­fied tech­nique, which requires some cal­cu­la­tions but no graph­ing. I’ll send you to the book for the some­what more pre­cise method, but here’s my eas­ier method: In a fourth col­umn, divide the rain­fall num­ber by the tem­per­a­ture and mul­ti­ply by 2. That’s where the math comes in to play.

Here’s my result for San Diego:

Month Rain­fall (mm) Tem­per­a­ture (Celsius) 2 x (Rainfall/Temperature)
Jan 55.6 14.1 7.890
Feb 41.3 14.7 5.62
Mar 49.9 15.3 6.52
Apr 19.8 16.6 2.39
May 4.8 17.8 0.54
Jun 1.9 19.3 0.2
Jul 0.5 21.6 0.05
Aug 2.1 22.5 0.19
Sep 4.7 21.8 0.43
Oct 8.6 19.8 0.87
Nov 29.5 16.6 3.56
Dec 35.4 14.1 3.62

Count up the num­bers in the fourth col­umn that are less than 1, and that’s your approx­i­mate hydric deficit num­ber. The higher the hydric deficit num­ber, the more severe your dry­ing con­di­tions. For the San Diego Air­port, the num­ber is 6. (If you have a month where the aver­age tem­per­a­ture is below freez­ing, for my over­sim­pli­fied method just throw out that month and con­sider that there’s min­i­mal hydric deficit.)

Now what do you with the num­ber? For one thing, you can use it to com­pare you grow­ing con­di­tions with the drought resis­tance code for a plant in Filippi’s book. For exam­ple, the matil­ija (“tree”) poppy (Rom­neya coul­teri) has a drought tol­er­ance rat­ing of 6–perfect for an unwa­tered gar­den in San Diego. By con­trast, Cean­othus ‘Ray Hart­man’ has a code of 4, and Hid­cote Blue laven­der (Lavan­dula angus­ti­fo­lia ‘Hid­cote Blue’) has a code of 3. These other plants would prob­a­bly sur­vive with­out sup­ple­men­tal water, but to look their best the cean­othus might ben­e­fit from a cou­ple months of occa­sional sup­ple­men­tal water­ing, and the lan­der maybe three. You can also use the num­ber to com­pare the dry­ing forces where you live other regions around you, or apply the num­ber to bet­ter under­stand your cli­mate in rela­tion to that of a plant’s origin.

For fun, I added four other sites in San Diego County. You can see how the county offers a huge num­ber of grow­ing con­di­tions, from dry coastal con­di­tions, moun­tain mead­ows, back­coun­try chap­ar­ral, and full-on desert.

City Hydric deficit
San Diego Airport 6
La Mesa 5–6
Cuya­maca 1
Campo 3
Bor­rego Springs 7


And then a few other cities in Cal­i­for­nia. You can see a gen­eral moist­en­ing the far­ther north you go, and a gen­eral dry­ing as you head east towards the deserts.

City Hydric deficit
Los Ange­les 6
San Bernardino 4–5
Vic­torville 6
Santa Bar­bara 5
Mon­ter­rey 4
San Jose 4–5
Santa Cruz 3
San Fran­cisco 4
Rich­mond 4
Sacra­mento 4–5
Fresno 5
Yosemite National Park 2
Eureka 1 2
Red­ding 2


I’d never played with map­ping in Google Maps, but thought this might be a fun first lit­tle project. I took the num­bers above and applied them to a map. The results are pretty impres­sive for some­thing that’s not hard to do. So far the blips are in Cal­i­for­nia only, but I might work on the map some more to include other loca­tions. Take a look…


View Hydric Deficit Map in a larger map

May 21 2009 | Categories: gardening • landscape | Tags: • • • • • • | 10 Comments »