2.1 inches a year is the design value for usable rain in the low-rain parts of the valley. The minimum uses the low end of the home-range rainfall, the driest country the species evolved in, so it is a floor. The typical value uses the middle of the range and is the planning number.
BEST AcademyIRR-0No extra waterHome range as dry as the valleyIRR-1Up to 2 in/yrNearly lives on rainIRR-22–4 in/yrLow extra waterIRR-34–8 in/yrModerate extra waterIRR-48–16 in/yrHigh extra waterIRR-5Over 16 in/yrFrom a humid climateIRR-PGroundwater plantGrows in washes over a water table; needs deep wateringIRR-XNot yet ratedGarden hybrid or source not confirmedExtra water in inches a year, by the climate where the plants grow wild.
| Home climate | Plants | Home rain | Minimum | Typical | Maximum | Group | Rain season |
|---|---|---|---|---|---|---|---|
| Sonoran - Lower Colorado River Valley / Baja (driest subdivision) | 4 | 2-5 in | 0.0 | 1.4 | 2.9 | IRR-0 | summer monsoon + winter storms |
| Mojave Desert - valley floor / bajada | 52 | 3-10 in | 0.9 | 4.4 | 7.9 | IRR-1 | winter-dominant |
| Mojave Desert - alkali / saline flat | 4 | 3-10 in | 0.9 | 4.4 | 7.9 | IRR-1 | winter + shallow water table |
| SW Asia / Middle East (Irano-Turanian) | 1 | 4-12 in | 1.9 | 5.9 | 9.9 | IRR-1 | winter rain, summer drought |
| Central Asia (Irano-Turanian steppe-desert) | 1 | 4-12 in | 1.9 | 5.9 | 9.9 | IRR-1 | winter/spring rain |
| Southern Africa (Karoo / summer-rain veld) | 2 | 4-24 in | 1.9 | 11.9 | 21.9 | IRR-1 | summer rain (Karoo) / winter (Cape) |
| Great Basin Desert (cold desert) | 4 | 6-12 in | 3.9 | 6.9 | 9.9 | IRR-2 | winter snow |
| Chihuahuan Desert | 23 | 6-20 in | 3.9 | 10.9 | 17.9 | IRR-2 | summer monsoon dominant |
| Mojave - Great Basin transition (upper elevation) | 16 | 8-20 in | 5.9 | 11.9 | 17.9 | IRR-3 | winter snow + spring melt |
| California Floristic Province (chaparral / coastal sage) | 7 | 10-25 in | 7.9 | 15.4 | 22.9 | IRR-3 | winter rain, summer drought |
| Australia (arid interior / SW Mediterranean) | 6 | 8-25 in | 5.9 | 14.4 | 22.9 | IRR-3 | variable; SW winter-rain |
| Sonoran - Arizona Upland | 5 | 12-15 in | 9.9 | 11.4 | 12.9 | IRR-4 | bimodal - half in summer monsoon |
| Mediterranean Basin | 14 | 12-30 in | 9.9 | 18.9 | 27.9 | IRR-4 | winter rain, summer drought |
| Great Plains (shortgrass / mixed prairie) | 1 | 14-24 in | 11.9 | 16.9 | 21.9 | IRR-4 | summer-rain dominant |
| Tamaulipan thornscrub / South Texas Plains | 4 | 16-30 in | 13.9 | 20.9 | 27.9 | IRR-4 | summer-rain, semi-arid not desert |
| East Asia (temperate, summer-rain) | 12 | 20-60 in | 17.9 | 37.9 | 57.9 | IRR-5 | summer monsoon, humid |
| Eastern North America (temperate humid) | 5 | 30-50 in | 27.9 | 37.9 | 47.9 | IRR-5 | year-round, no dry season |
| Mojave Desert - wash / riparian | 22 | 3-10 in | not sizable | - | - | IRR-P | winter + groundwater |
| Sonoran - wash / xeroriparian | 7 | 3-20 in | not sizable | - | - | IRR-P | bimodal + groundwater |
| Multi-desert SW riparian (all four SW deserts) | 1 | 3-20 in | not sizable | - | - | IRR-P | bimodal + groundwater |
| Hybrid / horticultural cultivar (no wild climate) | 12 | n/a | n/a | - | - | IRR-X | n/a |
| UNRESOLVED - needs source verification | 5 | n/a | n/a | - | - | IRR-X | n/a |
BEST Academy, Minimum Supplemental Irrigation by Evolutionary Origin, version 3, September 2026 (208 plants), built on the BEST Academy climate-group matrix.