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Persistence and memory time scales in root-zone soil moisture dynamics

Persistence and memory time scales in root-zone soil moisture dynamics
Persistence and memory time scales in root-zone soil moisture dynamics
The memory timescale that characterizes root-zone soil moisture remains the dominant measure in seasonal forecasts of land-climate interactions. This memory is a quasi-deterministic timescale associated with the losses (e.g. evapotranspiration) from the soil column and is often interpreted as persistence in soil moisture states. Persistence, however, represents a distribution of time periods where soil moisture resides above or below some prescribed threshold, and is therefore inherently probabilistic. Using multiple soil moisture datasets collected at high resolution (sub-hourly) across different biomes and climates, this paper explores the differences, underlying dynamics, and relative importance of memory and persistence timescales in root-zone soil moisture. A first-order Markov process, commonly used to interpret soil moisture fluctuations derived from climate simulations, is also used as a reference model. Persistence durations of soil moisture below the plant water-stress level (chosen as the threshold), and the temporal spectrum of up- and down-crossings of this threshold, are compared to the memory timescale and spectrum of the full time series, respectively. The results indicate that despite the differences between meteorological drivers, the spectrum of threshold-crossings is similar across sites, and follows a unique relation with that of the full soil moisture series. The distribution of persistence times exhibits an approximate stretched exponential type and reflects a likelihood of exceeding the memory at all sites. However, the rainfall counterpart of these distributions shows that persistence of dry atmospheric periods is less likely at sites with long soil moisture memory. The cluster exponent, a measure of the density of threshold crossings in a time frame, reveals that the clustering tendency in rainfall events (on-off switches) does not translate directly to clustering in soil moisture. This is particularly the case in climates where rainfall and evapotranspiration are out of phase, resulting in less ordered (more independent) persistence in soil moisture than in rainfall.
soil moisture, persistence, memory, climate, rainfall
0043-1397
1427-1445
Ghannam, Khaled
378e2d29-7476-40b2-8a17-0aa6eaaebfa8
Nakai, Toro
6dc1b424-2b07-4bea-b932-b94399b12ff4
Paschalis, Athanasios
e7626e9f-172b-4da2-882c-bddb219f3fb6
Oishi, Christopher A.
9f912b78-0159-4116-b47d-fae58aa8fe88
Kotani, Ayumi
fb93bbf4-de64-4db8-89cb-275b51baa9f1
Igarashi, Yasunori
f7a2298b-8fd7-4053-af91-3856d4c09b8b
Kumagai, Tomo'omi
02e35912-1d6b-41d3-b528-5af79deeb9df
Katul, Gabriel G.
5f0cb2ab-2d91-4cc4-9ac2-e5c2eaf928f9
Ghannam, Khaled
378e2d29-7476-40b2-8a17-0aa6eaaebfa8
Nakai, Toro
6dc1b424-2b07-4bea-b932-b94399b12ff4
Paschalis, Athanasios
e7626e9f-172b-4da2-882c-bddb219f3fb6
Oishi, Christopher A.
9f912b78-0159-4116-b47d-fae58aa8fe88
Kotani, Ayumi
fb93bbf4-de64-4db8-89cb-275b51baa9f1
Igarashi, Yasunori
f7a2298b-8fd7-4053-af91-3856d4c09b8b
Kumagai, Tomo'omi
02e35912-1d6b-41d3-b528-5af79deeb9df
Katul, Gabriel G.
5f0cb2ab-2d91-4cc4-9ac2-e5c2eaf928f9

Ghannam, Khaled, Nakai, Toro, Paschalis, Athanasios, Oishi, Christopher A., Kotani, Ayumi, Igarashi, Yasunori, Kumagai, Tomo'omi and Katul, Gabriel G. (2016) Persistence and memory time scales in root-zone soil moisture dynamics. Water Resources Research, 21 (2), 1427-1445. (doi:10.1002/2015WR017983).

Record type: Article

Abstract

The memory timescale that characterizes root-zone soil moisture remains the dominant measure in seasonal forecasts of land-climate interactions. This memory is a quasi-deterministic timescale associated with the losses (e.g. evapotranspiration) from the soil column and is often interpreted as persistence in soil moisture states. Persistence, however, represents a distribution of time periods where soil moisture resides above or below some prescribed threshold, and is therefore inherently probabilistic. Using multiple soil moisture datasets collected at high resolution (sub-hourly) across different biomes and climates, this paper explores the differences, underlying dynamics, and relative importance of memory and persistence timescales in root-zone soil moisture. A first-order Markov process, commonly used to interpret soil moisture fluctuations derived from climate simulations, is also used as a reference model. Persistence durations of soil moisture below the plant water-stress level (chosen as the threshold), and the temporal spectrum of up- and down-crossings of this threshold, are compared to the memory timescale and spectrum of the full time series, respectively. The results indicate that despite the differences between meteorological drivers, the spectrum of threshold-crossings is similar across sites, and follows a unique relation with that of the full soil moisture series. The distribution of persistence times exhibits an approximate stretched exponential type and reflects a likelihood of exceeding the memory at all sites. However, the rainfall counterpart of these distributions shows that persistence of dry atmospheric periods is less likely at sites with long soil moisture memory. The cluster exponent, a measure of the density of threshold crossings in a time frame, reveals that the clustering tendency in rainfall events (on-off switches) does not translate directly to clustering in soil moisture. This is particularly the case in climates where rainfall and evapotranspiration are out of phase, resulting in less ordered (more independent) persistence in soil moisture than in rainfall.

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More information

Accepted/In Press date: 2 February 2016
e-pub ahead of print date: 5 February 2016
Published date: 28 February 2016
Keywords: soil moisture, persistence, memory, climate, rainfall
Organisations: Water & Environmental Engineering Group

Identifiers

Local EPrints ID: 388484
URI: https://eprints.soton.ac.uk/id/eprint/388484
ISSN: 0043-1397
PURE UUID: 8005db38-b50d-4021-b5c6-c8112490cc8f
ORCID for Athanasios Paschalis: ORCID iD orcid.org/0000-0003-4833-9962

Catalogue record

Date deposited: 26 Feb 2016 13:58
Last modified: 02 Dec 2019 20:22

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