Lit-review precipitation + drying methodology + interpretation backing (2/3)

[NewGraphEnvironment]

Problem

Vignette interpretation paragraphs in vignettes/peace-fwcp.Rmd and
vignettes/kootenay-lake.Rmd make defensible-sounding claims about
precipitation departure and "drying" — falling annual precipitation,
rising VPD/evapotranspiration, declining soil moisture, "soils drying
due to both ↓P and ↑ET" (the v0.1.1 finding) — but currently land on
zero peer-reviewed citations. For FWCP fish-passage reporting
context, these need the same cited backing #53/#54 gave Snowpack
and #58/#60 gave Temperature.

Scope

Second of three sequential climate-departure lit reviews covering
the non-snow vignette sections (3-split: temperature [done #58],
precip+drying [this issue], interpretation framing).
Mirrors the #58 / #54 / v0.1.7 pattern verbatim:

• Targeted lit search → ~10 papers
• Add to NewGraphEnvironment/climate Zotero collection (key
  8MH9LCC9), PDFs first per /lit-search policy
• Build data/rag/precip_drying_methodology.duckdb ragnar store
• Mine for methodology quotes; write findings.md with "cite this
  for that" map
• Vignette [@key] insertion happens on a downstream branch, not here

Topics to cover

1. Precipitation trend methodology — long-record analyses,
   homogenization, treatment of trace events
2. Heavy / extreme precipitation — intensity-frequency-duration,
   anthropogenic attribution
3. Orographic / mountain precipitation — rain-shadow gradients,
   Selkirk-Purcell type contrasts (relevant for Kootenay vignette)
4. Vapor pressure deficit (VPD) + relative humidity — atmospheric
   evaporative demand driver
5. Soil moisture as integrative drought signal — supply-demand
   balance, temperature acceleration of drying
6. ERA5-Land precip / soil-moisture validation — bias structure
   (parallel to the snow-rag's Kouki 2023 for SWE)
7. Drought-fish linkage — how summer-flow / soil-moisture deficit
   propagates to thermal habitat for salmonids

Starting candidate list (will refine)

Citation key (proposed)	Topic / Why
donat_etal2013 HadEX2	Global temp + precip extremes dataset (J Geophys Res)
min_etal2011	Anthropogenic contribution to extreme precipitation (Nature)
mekis_vincent2011	Adjusted daily Canadian precipitation dataset (Atmos-Ocean)
daly_etal2008 PRISM	Physiographic mapping of climate over US — orographic methods
mass_etal2002	Orographic precip processes PNW (Mon Wea Rev / J Hydromet)
williams_etal2020	Anthropogenic warming → North American megadrought (Science)
ficklin_novick2017	Globally rising VPD (J Climate)
grossiord_etal2020	Plant + ecosystem responses to rising VPD (New Phytol)
trenberth_etal2014	Global warming + drought changes (Nature Clim Chg)
marvel_etal2019	20th-century hydroclimate changes (Nature)
sheffield_wood2008	Global drought trends + variability (J Climate)

Existing items in climate collection to screen first

19 items already there. From #58 reuse pool, several are also
relevant on the precip/drying side:
munoz-sabater_etal2021ERA5Landstateoftheart (ERA5-Land soil-moisture
validation), dierauer_etal2020Climatechange (BC ecoregion drought),
islam_etal2019Quantifyingprojected (Fraser flow regime — was set
aside in #58 as Issue-2 scope), warkentin_etal2022Lowsummer (BC
summer flow + chinook), vincent_etal2018ChangesCanadas (Canada
precip trends are in the same paper as the temp trends used in #58).

Already in snow rag (cross-rag, no re-add):
knowles_etal2006SnowfallVersus (rain-vs-snow phase shift).

Phases (mirror #58 task_plan)

1. Targeted lit search — candidate list, DOI confirmation, OA / RG access
2. Add to NewGraphEnvironment/climate — PDFs first
3. Build ragnar DuckDB at data/rag/precip_drying_methodology.duckdb
4. Mine the store for methodology quotes (8 query topics)
5. Synthesis + citation map in findings.md
6. PR — squash, NEWS bullet, v0.x.y patch release

Out of scope

• Temperature methodology — covered by Lit-review temperature-departure methodology + interpretation backing #58/Temperature-departure lit review: rag-build + 10 papers + cite map (1/3) #60
• Snowpack methodology — covered by Snowpack-departure methodology lit review: Zotero + ragnar + cited vignette #53/Snowpack-departure methodology lit review: rag-build + 11 papers + citation map #54
• Interpretation framing — Issue 3 of the 3-split (forthcoming)
• BEC zone shifts — Lit-review BEC (biogeoclimatic ecosystem classification) zone shifts under climate change [tracker] #59 tracker (post-3-split)
• Vignette citation insertion — downstream consumer branch
• Original methodology research / new metric proposals