Terrain analysis and steady-state hydrological modelling of a small catchment in southern China

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1 2005, 14(5): Ecology nd Environment E-mil: Terrin nlysis nd stedy-stte hydrologicl modelling of smll ctchment in southern Chin HUANG Zhi-hong 1, 2, ZHOU Guo-yi 2*, ZHOU Gung-yi 3, MORRIS Jim 4 SILBERSTEIN Richrd 5, WANG Xu 2, 6 1. Yingdong School of Bioengineering, Shogun University, Shogun , Chin; 2. South Chin Botnicl Grden, Chinese Acdemy of Sciences, Gungzhou , Chin; 3. Reserch Institute for Tropicl Forestry, Gungzhou , Chin; 4. School of Forest nd Ecosystem Science, The University of Melourne, PO Box 137, Heidelerg Victori, 3084, Austrli; 5. CSIRO Lnd nd Wter, PO Wemley WA, 6913, Austrli; 6. Grdute School of the Chinese Acdemy of Sciences, Beijing , Chin Astrct: Mny hydrologicl chrcteristics of ctchment cn e inferred from its topogrphy. The eco-hydrologicl model, Topog, uses sophisticted nlysis of topogrphy to descrie the hydrologicl chrcteristics of ctchment in detil. This pper descries n integrted terrin nlysis nd stedy stte hydrologicl modelling study of smll forest ctchment on Leizhou Peninsul, southern Chin using Topog. The terrin nlysis ws sed on DEM (digitl elevtion model) of the centrl prt of the peninsul including the upper vlley of the Nndu River. The sic hydrologic chrcteristics defining the Jiji ctchment were ctchment oundry, high points nd sddles, clculted ridges nd strems, nd n element network seprting the ctchment into lrge numer of reltively uniform units for modelling. The topogrphic ttriutes of ech element were clculted utomticlly, including slope, spect, upslope contriuting re nd potentil incident solr rdition. The slope of the ctchment ws reltively low: the difference etween slopes of most elements ws in the rnge of 2.8~5.7 degree, or less thn 2.8 degree. The generl description of the Jiji ctchment provided y Topog included totl ctchment re of 0.63 km 2 nd verge mount of incident rdition of 44, 25, nd 34 MJ m -2 d -1 for summer, winter nd equinoxes, respectively. The ctchment convergence index nd stedy-stte wetness indices (WI) of the elements of the Jiji experimentl ctchment with nd without solr rdition-weighting were lso otined. From stedy-stte dringe flux modelling, we otined distriution of WI cross the ctchment. By setting different prmeter vlues of uniform dringe flux, the mpped simultions of WI over the ctchment indicted tht the igger the uniform dringe flux ws, the higher the WI would e.we modelled rdition-weighted dringe index t different vlues of uniform trnsmissivity (T), different shded soil frction (Es), nd different uniform rinfll (R). The result illustrted tht the mpped distriution of WI vries s consequence of these different dt inputs. The distriution of WI ws strongly ffected y T vlues which indicted tht soil wetness within some strem zones might extend more widely, given igger T vlue. Conversely, lower vlues of T resulted in more uniform sptil distriution of WI over the ctchment. Modelled results lso vried with shded frction, which indicted tht smll increse in solr rdition would result in sptilly different distriution of soil moisture content over the ctchment. Finlly, we mde comprison etween set of uniform rinfll vlues nd found tht Topog predicted the expected trend tht soil moisture within the ctchment incresed with incresing uniform rinfll vlues. Key words: euclyptus plnttions; Topog modelling; wetness index; Leizhou Peninsul CLC numer: Q Document code: A Article ID: Reforesttion is importnt in Chin, where lrge res of euclyptus plnttions hve een estlished in Gungdong nd other southern provinces, nd selection nd reeding hve chieved high growth rtes. The lrge scle estlishment of euclyptus plnttions in southern Indi [1], South Afric [2, 3] nd other countries hve een ssocited with depletion of wter resources [4]. Therefore, it is importnt to understnd the hydrologicl effects of plnttions in Chin. A four-yer study ws conducted, eginning in 1999, on the Leizhou Peninsul of western Gungdong, Chin to mesure wter use of euclyptus plnttions. Hydrologicl modelling, in conjunction with field experimenttion, cn e very useful in understnding ctch- (2002CB1115) KZCX1-SW-01-01A3 KSCX2-SW-120 FST97/ Tel E-mil: zhihong@scg.c.cn * E-mil: gyzhou@scg.c.cn

2 701 ment hydrology. Hydrologicl chrcteristics including soil moisture nd erosion hzrd index re redily clculted y the distriuted prmeter eco-hydrologicl Topog model [5]. Clcultion of the erosion index entils considertion of most of the prmeters ffecting the erosion processes, including locl nd source re topogrphy, soil hydrulic nd strength properties, vegettion cover nd climtic fctors [6]. This pper descries the ppliction of the Topog model to some of the project dt to mke preliminry nlysis of the stedy-stte hydrologicl properties of smll typicl ctchment. Further ppers will report the dynmic modelling of stremflow nd groundwter rechrge in the ctchment, in reltion to climte nd soil conditions nd vegettion mngement. 1 Study site The plnttions re locted t Jiji Forest Frm, the soil of which is clssified s deep red cly soil, developed form slt, with high ggregte stility nd low fertility within the Jiji demonstrtion ctchment re (20 54 N, E). The site is on flt to undulting terrin typicl of the peninsul lowlnds. Leizhou peninsul, Gungdong Province, hs tropicl climte, with long term monthly men tempertures of round 28 in July nd 16 in Jnury. Annul precipittion vries from mm in the south to mm in the north of the peninsul nd nnul vrition is high. Over 80% of the rinfll occurs etween April nd Septemer, up to hlf of this in typhoons which occur up to seven times per yer. At the study site, the men dily mximum tempertures were 31º in July nd 20º in Jnury, nd men dily minimum tempertures were 22º nd 13º in July nd Jnury, respectively. The monitored plnttions were Euclyptus urophyll plnted in mid-1996 (2 m 1.5 m spcing). The investigtion of reltively young plnttions is pproprite s the usul rottion period for plnttions in the re is 4 yers. At the experimentl plots, the norml lnd use includes euclyptus plnttions grown on short rottions for woodchip nd fuel wood production; sugr cne; nd other griculture including pinepples, nns, melons nd vegetles. 1.1 The Topog hydrologicl model pckge Developed y CSIRO Lnd nd Wter, Austrli [7 9] nd pplied in mny situtions [5, 10 12], Topog descries how wter moves through lndscpes; over the lnd surfce, into the soil, through the soil nd groundwter nd ck to the tmosphere vi evpotrnspirtion. Conservtive solute movement nd sediment trnsport re lso simulted. The primry strength of Topog is tht it is sed on sophisticted digitl terrin nlysis model, which ccurtely descries the topogrphic ttriutes of three-dimensionl lndscpes. It is verstile nd comprehensive hydrologicl model, intended for ppliction to smll ctchments (up to 10 km 2, nd generlly smller thn 1 km 2 ). Topog is terrin nlysis-sed hydrologicl modelling pckge which cn e used to: descrie the topogrphic ttriutes of complex three dimensionl lndscpes; predict the sptil distriution of stedy stte wterlogging, erosion hzrd nd lndslide risk indices; simulte the trnsient hydrologic ehviour of ctchments, nd how it is ffected y chnge of ctchment vegettion; imitte the growth of vegettion nd how it impcts on the wter lnce; model solute movement through the soil; model sediment movement over the soil surfce. Topog softwre is ville free of chrge from the Topog Internet site, The progrms re usully compiled in Unix or Linux operting system. 1.2 Methods For the development of DEM, t scle, 2.5 m contour intervl topogrphic mp (Tngji Community) ws digitized using Didger (Golden Softwre Inc, USA) softwre, nd the output file ws edited using Microsoft Excel to produce Jiji.xyz, dt file in the required formt for input to Topog. Topog ws compiled from downloded source progrms in Gentus Linux 6.2 environment on Pentium (800 MHz) computer. The Demgen nd Element utilities were used to crete n element network using contour intervl of 2 m. The-Sumtr, -Prms nd -Xhistog progrms were used to determine nd compre the hydrologicl properties of ech element, nd the progrm-simul ws pplied to clculte stedy-stte simultions of wetness index (WI). The wetness index [5, 12] in Topog is clculted s: WI = 1 MT q( x, y) da 1 where M is the locl slope (m/m, dimensionless), T is the locl trnsmissivity of the soil profile m 2 d -1, nd the region of integrtion is the upslope ctchment re per unit length of contour. A is element re m 2 q is net susurfce dringe flux (m d -1 ), sptilly vrying quntity. WI depends on locl chrcteristics of soil nd terrin, nd on integrted dringe chrcteristics from upslope. All of these cn e sptilly vrile. Chnges in ny of them due to lnd use will result in chnge in WI. This is the sis for the modelling techniques pplied in Topog. 2 Results As mny other reserchers hve demonstrted, the eco-hydrologicl model Topog focuses on descriing the complex interreltionship etween topogrphy, soils, climte nd vegettion. Topog is sophisticted in hndling of complicted contour topogrphy, which egins with digitl elevtion

3 model DEM of the re to e modelled. 2.1 Terrin Anlysis using Topog Almost ll the nlysis of ctchment chrcteristics is sed on the topogrphy [13]. Contours of the hillslope, nd the network of computtionl elements re shown in Figure 1. The sic hydrologic chrcteristics of the Jiji ctchment re shown, including ctchment oundry, high points nd sddles, clculted ridges nd strems, nd element network. Once the surfce discretistion hs een completed, suite of lgorithms re used to perform rnge of nlyses on ech element in the c Fig. 1 Full element network for Jiji demonstrtion ctchment, illustrting nlysis of complex topogrphy. Results shown for () ctchment oundry nd high points (H) nd sddle points (S), nd () elements derived from Topog-element, plotted t 2 m intervls, showing derived strem chnnels (lue) nd derived dividing ridge (red); nd (c) possile contour-sed runoff trjectories nd strems y Topog modelling clculted include slope, spect, upslope contriuting re nd potentil solr rdition. All the results re stored in vrious files for ccess during lter hydrologic simultions. The red line in Figure 1() shows clculted ridge etween two vlleys or strems. The slope of the ctchment is low, which is in ccordnce with the generl topogrphy of Leizhou Peninsul. The slopes of most elements re in rnge of 2.8~5.7 degree, or less thn 2.8. Figure 2() shows vrition in spect cross the ctchment, which would impose effects on distriution of solr rdition over the ctchment. computed network. The topogrphic ttriutes utomticlly We now hve generl ide of the hydrologicl chrcteristics of the Jiji ctchment using Topog s nlysis of element ttriutes, such s ctchment convergence index, winter nd summer rdition. The totl re of the ctchment is 0.63 km 2 nd it hs een divided into 751 elements in ll. The lrgest element hs n re of m 2 while the smllest element is 139 m 2 with verge re of 844 m 2 (Figure 1()). As shown in Figure 2(), the slope is gentle nd the mximum slope is 6.3 degree with verge slope of 3.1. The mximum, the minimum nd the verge hill slope length re 141 m, 9 m nd 31 m, respectively. The mximum, verge, nd minimum Fig. 2 Distriution of spect nd slope over Jiji ctchment deduced y Topog from the DEM, shown () for spect nd () for slope mount of summer rdition re 44 MJ m -2 d -1, 44 MJ m -2 d -1, nd 42 MJ m -2 d -1, respectively. Those of winter rdition re 27 MJ m -2 d -1, 25 MJ m -2 d -1, nd 22 MJ m -2 d -1, respectively; while those of equinox rdition re 35 MJ m -2 d -1, 34 MJ m -2 d -1, 33 MJ m -2 d -1, respectively (Figure 3). 2.2 Stedy-stte modelling of Wetness Index for the Jiji Ctchment Topog requires the vlues of environmentl stte vri-

4 703 c Fig. 3 The distriution of solr rdition over the ctchment shown () for the winter, () for summer, nd (c) for equinox les such s soil depth, hydrulic conductivity nd vegettion cover, ll of which re likely to e sptilly distriuted in the ctchment. With the results from Topog modelling, we otined distriution of WI of the ctchment. By setting different prmeter vlues of uniform dringe flux (the stedy downslope movement of wter through the soil of the ctchment), the simul- tions shown in Figure 4 were derived under stedy-stte conditions, ut trnsient simultions cn lso e modelled. The mpped distriution of WI over the Jiji Ctchment is shown in Figure 4 nd 4, indicting tht the igger the uniform dringe flux is, the lrger scle the WI will e. Such mps re useful in deciding whether loclized plnted sites re similr to ech other for comprison of tree growth potentil. Fig. 4 Distriution of WI in the Jiji ctchment () for uniform dringe flux t the rte of 0.3 mm d -1 nd () for uniform dringe flux t the rte of 1.0 mm d Rdition-weighted dringe index If the mount of solr rdition is tken into ccount in the modelling, there should e nother pttern of distriution of WI over the ctchment, ecuse differences in rdition mong elements ffect the loss of wter y evpotrnspirtion. We modelled rdition-weighted dringe index t different vlues of uniform trnsmissivity (T). The results illustrte tht t different vlues of shded frction, different uniform T, nd different uniform rinfll re pplied, the mpped distriution of WI vries s consequence of these different dt inputs. The distriution of WI t different uniform T vlues would hve different responses even t the sme uniform rinfll nd other prmeter inputs(see Tle 1). Distriution of WI in Figure 5 nd 5 is just in this cse to show different resulting clsses in the ctchment re. In Figure 5, WI on 83% of the re rnged from 0.25~0.50, nd 16% of re rnged from 0.50~1.0, respectively. By comprison, the two indices s shown in Figure 5 re 60% nd 36%, respectively, reflecting wetter ctchment when uniform rinfll is greter. And this difference showed tht redistriution of soil moisture is strongly ffected y soil trnsmissivity vlues. Soil wetness (indictive of potentil wterlogging) within some strem res my extend fster to igger scle in djcent elements, given igger uniform T vlue s shown in Figure 5. The uniform T vlue of 3.0 m 2 d -1 nd uniform rinfll vlue of 4.0 mm d -1 we re next set to the sme vlues two times for modelling of rdition-weighted dringe index, while the shded frction prmeter ws set to 0.02 nd 0.05, respectively. The differences etween them re shown in Figure 5 nd Figure 5c. In Figure 5c the mpped distriution of WI ws 12% of re in the rnge from to 0.25, 81% of re in the rnge from 0.25 to 0.50, nd 7% of the re in the rnge from 0.50 to 1.0. This difference etween them indicted tht incresed solr rdition would result in sptilly different re-distriution of

5 Tle 1 Prmeter vlues for stedy-stte modelling of wetness indices T(uniform R(uniform rinfll)/ Es(shded frction trnsmissitvity)/ (m 2 d (mm d -1 ) energy) -1 ) Applied to Figure Figure Figure 5c Figure 5d Figure 5e soil moisture content. Finlly, we compred uniform T vlue of 4.0 m 2 d -1 nd uniform rinfll of 5.0 mm d -1, with uniform T vlue of 2.0 m 2 d -1 nd uniform rinfll of 5.2 mm d -1. The resulting s ptil differences in rdition-weighted dringe index re shown in Figure 5d nd Figure 5e. The mpped distriution of soil moisture content WI in Figure 5d shows 35% of the re in the rnge from 0.25 to 0.50, 62% of re in the rnge from 0.50 to 1.0. In Figure 5e with lower trnsmissivity nd slightly higher rinfll, the distriution of WI ws 11% nd 86% of the re in c c d e Fig. 5 Distriution of solr rdition-weighted WI over the ctchment in different conditions. Shown () for uniform T ssumed s 2.0 m 2 d -1 () for T 3.0 m 2 d -1 ; nd (c) for shded frction of 0.05 with the sme uniform T vlue s (); nd (d) for uniform rinfll of 5.0 mm d -1 nd uniform T of 4.0 m 2 d -1 ; nd (e) for uniform rinfll of 5.2 mm d -1-1 nd uniform T of 2.0 m 2 d the corresponding clsses. Results from this modelling indicte tht lower soil trnsmissivity leds to more uniform sptil distriution of WI over the ctchment, s shown in Figure 5d nd Figure 5e. We mde comprison of prmeter dt input for Figure 5e with the rest of the results from 5 to 5d, we found tht there is trend tht soil moisture within the ctchment increses with uniform rinfll vlue. 3 Conclusions The hydrologicl processes t ctchment scle re driven y climte, soil nd vegettion fctors, which re not fully ddressed in this preliminry nlysis. The results otined ove should provide preliminry pprecition of influences of topogrphy, rinfll, rdition nd soil trnsmissivity on hydrologicl responses. The pproprite niche for physicl models such s Topog is most likely to simulte ctchment ehviour under lterntive mngement systems. Ctchment mngement, for most purposes, requires good understnding of the system s response to chnge. Topog is useful tool to predict tht response [12]. The wetness index (WI) s clculted y Topog cn serve s useful dignosis to lnd mngers concerned with protecting lndscpes sensitive to disturnce. With the id of Topog, we cn provide lterntive cost-effective solutions for decision-mking y mngers nd locl residents. Those who would pln the lnd-use should hve some ide of the res within the ctchment which re most suject to high nd low soil moisture conditions. At the sme time, the results from DEM nd stedy-stte modelling could provide sis for more detiled dynmic nlysis of ctchment runoff nd groundwter rechrge in reltion to climte nd vegettion chnges.

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