> ## Documentation Index
> Fetch the complete documentation index at: https://badixth-dc85e378.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Drought Stress in Rubber

> How drought manifests on Hevea rubber, why NDWI and CWSI lead NDVI on closed canopies, and the field actions to take on immature and mature stands.

Rubber (*Hevea brasiliensis*) tolerates a defined dry season (2 to 4 months) but suffers on prolonged drought, especially during El Nino years. Immature blocks (year 1 to 3) die outright; mature tapped blocks show reduced latex flow, delayed refoliation, and higher TPD incidence in the following season.

## Cause

* **El Nino dry spells** in Malaysia (2015-16, 2019, 2023) with rainfall below 100 mm/month for 3+ months.
* **Sandy or shallow soils** with poor water-holding capacity.
* **Slope aspect** on hilly blocks (west and south faces dry faster).
* **Immature blocks** (year 0 to 3) with shallow root systems and open canopy.
* **Over-tapping** compounds drought stress by depleting stored photosynthates.

## Effect

| Stand age                  | Effect                                                                                     |
| -------------------------- | ------------------------------------------------------------------------------------------ |
| 0 to 1 year                | Seedling wilt and death; establishment failure                                             |
| 1 to 3 years               | Growth arrest, defoliation, dieback of tender shoots                                       |
| 3 to 5 years (pre-tapping) | Delayed canopy closure, reduced girth increment                                            |
| 5+ years (tapped)          | Latex flow drop (10 to 40%), lower DRC, delayed wintering recovery, higher TPD next season |

Field signs:

* Wilting of young leaves in immature blocks
* Premature leaf yellowing and drop out of wintering window
* Latex "pre-coagulation" in the tapping cup (latex clots before collection)
* Reduced latex flow duration (drying up within 1 to 2 hours instead of 3 to 4)
* Bark shrinkage and cracking on the tapping panel

## Satellite signals

**NDWI and CWSI lead NDVI by 2 to 4 weeks on closed rubber canopies.** NDVI saturates on mature rubber; use it only for immature blocks or catastrophic drought.

| Index                   | What to watch             | Threshold                          |
| ----------------------- | ------------------------- | ---------------------------------- |
| NDWI                    | Drop below block baseline | -0.05 vs same block year-over-year |
| CWSI                    | Rise above baseline       | > 0.4 sustained for 2 weeks        |
| NDRE                    | Decline on tapped blocks  | -0.03 vs baseline                  |
| NDVI                    | Immature-block collapse   | -0.10 sustained                    |
| LST (land surface temp) | Canopy warming            | > 3 C above block baseline         |

## Action

<Steps>
  <Step title="Confirm the pattern">
    Cross-check NDWI/CWSI with local rainfall records. Walk the block using the [Field Walk Protocol](/guides/Crop/Rubber/field-walk-protocol) and look for wilting, pre-coagulation, and panel cracking.
  </Step>

  <Step title="Immature blocks: irrigate or mulch">
    Ring-mulch young trees with legume cuttings or EFB. If irrigation is available, prioritize year 1 to 2 blocks. Expect 20 to 40% mortality on unwatered year-1 blocks after 3 months of severe drought.
  </Step>

  <Step title="Tapped blocks: reduce tapping intensity">
    Switch from d/2 (every 2 days) to d/3 or d/4 during severe drought. Suspend stimulation (ethephon) until refoliation. Continued heavy tapping under drought is the top cause of TPD in the following season.
  </Step>

  <Step title="Cover the ground">
    Maintain legume cover crops on immature blocks. On mature blocks, leaf litter is the main mulch; do not rake or burn.
  </Step>

  <Step title="Monitor recovery">
    After rain returns, expect 4 to 8 weeks for NDWI/NDRE to recover. Latex flow recovers within 2 weeks. If flow does not recover on individual trees, suspect [TPD](/guides/Crop/Rubber/biotic/tpd).
  </Step>
</Steps>

## Recommended alert settings

| Trigger             | Condition                                 | Severity |
| ------------------- | ----------------------------------------- | -------- |
| NDWI drop           | -0.05 vs baseline sustained 14 days       | Medium   |
| CWSI rise           | > 0.4 sustained 14 days                   | Medium   |
| Rainfall deficit    | \< 100 mm/month for 2+ consecutive months | Medium   |
| Rainfall deficit    | \< 100 mm/month for 3+ consecutive months | High     |
| Immature-block NDVI | -0.10 sustained 21 days on year 0-3 block | High     |

## Related

* [Rubber Diagnosis](/guides/Crop/Rubber/rubber-diagnosis)
* [Tapping Panel Dryness](/guides/Crop/Rubber/biotic/tpd)
* [Rubber ecosystems (NDVI baselines)](/guides/Crop/Rubber/rubber-ecosystems)